All too often, a choice that seems sustainable(可持续的)turns out on closer examination to be problematic. Probably the best example is the rush to produce ethanol(乙醇) for fuel from corn. Corn is a renewable resource —you can harvest it and grow more, almost limitlessly. So replacing gas with corn ethanol seems like a great idea. 

    One might get a bit more energy out of the ethanol than that used to make it, which could still make ethanol more sustainable than gas generally, but that's not the end of the problem. Using corn to make ethanol means less corn is left to feed animals and people, which drives up the cost of food. That result leads to turning the fallow land –including, in some cases, rain forest in places such as Brazil—into farmland, which in turn gives off lots of carbon dioxide (CO) into the air. Finally, over many years, the energy benefit from burning ethanol would make up for the forest loss. But by then, climate change would have progressed so far that it might not help.

    You cannot really declare any practice “sustainable” until you have done a complete life-cycle analysis of its environmental(环境的) costs. Even then, technology and public keep developing, and that development can lead to unforeseen and undesired results. The admirable goal of living sustainably requires plenty of thought on an ongoing basis.

    Audrey Hepburn won an Academy Award as Best Actress for her first major American movie，Roman Holiday，which was released in 1953．But she is remembered as much for her aid work as for her acting．

    Born in Belgium in 1929，Audrey's father was British and her mother was Dutch．Audrey was sent to live at a British school for part of her childhood．During World War II，she lived and studied in the Netherlands. Her mother thought it would be safe from German attacks．Audrey studied dance as a teenager and during college when she returned to London after the war．But she realized she wasn't going to be a ballerina （芭蕾舞女演员）．So she began taking acting parts in stage shows．Later she began to get small parts in movies．

    But it was Audrey Hepburn's move to America that brought her true fame．In 1951 she played the character"Gigi"in the Broadway play of the same name to great critical praise．Two years later，Roman Holiday made her a star at the age of 24．

    Audrey made more than 25movies．Among her most popular roles was Holly Golightly in Breakfast at Tifany's in 1961．Three years later she played Eliza Doolittle in My Fair Lady．

    She was married two times and had one son by each husband．In 1989，the UN Children's Fund named Audrey a goodwill ambassador．She travelled all over the world in support of UNICEF （联合国儿童基金会） projects．The UN agency said she was a tireless worker．She often gave 15interviews a day to gain money and support for UNICEF projects．

    Audrey Hepburn often said her loyalty to UNICEF was the result of her experiences as a child during World War II．She said she knew what it was like to be starving and to be saved by international aid．She was a goodwill ambassador until her death in 1993from colon cancer．

    “Why do birds sing?” I asked a group of children. “To attract metes(配偶)!” answered a boy. A 4-year-old girl added shyly, “birds sing because they are happy, and they want to make us happy, too.” I think both answers are probably true in a way. If birds sing happily, it shows all is well in the forest: no predators (捕食者) are present, and the birds can feed, sing and do other bird things. The sound of a relaxed robin(知更鸟) singing is easy to understand because it makes us feel this way too—it's a sound that lifts our own hearts when we hear it.

    What happens when a predator comes into their areas? The nearby birds quickly give an alarm call, a short sound that warns, “Danger is coming!” On hearing the sound, the other birds stop singing and fly for cover, the deer lift their heads and stop moving and the rabbits run into the bushes. Suddenly, the forest becomes silent.

    When humans walk through the woods in a hurry or while talking with a friend, we don't see much wildlife. We may think that there aren't many animals living there, but more likely, they know we are coming and hide minutes before our arrival. But if we learn to avoid setting off alarms, we can see many more wild animals without frightening them. To practice, find a place in a park and sit quietly for an hour. Take note as the birds begin to relax and sing again. When I do this, I am always surprised at the wonderful thing animals will do when humans simply stay still long enough.

    The United States of America is faced with serious traffic problems. Too many cars travel on the freeways to and from the city. This heavy traffic causes delays and lots of pollution from cars that are using their engines but not moving anywhere. Many cars going nowhere is called a "traffic jam".

    While many cities in the United States have traffic problems, some of the worst are in Los Angeles, California, which is a big city with a large downtown area. One of the reasons why there is so much traffic is that few people use public transportation to get around.

    Experts say one possible solution to the traffic problems in Los Angeles would be to make people want to use the public transportation system of buses instead of their own cars. People say the buses should be made easier, faster, less costly and more dependable than driving a car.

    One suggestion is to reduce the cost of taking the bus. Also, Los Angeles could make the buses easier for people to use. The buses could travel to more places in the city. And there could be more special lanes on the freeways that only buses could drive in. These changes would make taking the bus faster and easier than driving a car.

    Other experts have said that the best way to ease traffic problems is to charge people money to drive on the freeways. These charges, or tolls, would be for the most crowded roads. The tolls would be in effect during the times of a day when most people drive to work and home. These times in the morning and late afternoon are called "rush hour". People would try to avoid driving on these roads, which would decrease the amount of traffic. Also, the money collected from these tolls could be used to improve the roads and public transportation system.

    The fiddler crab (蟹) is a living clock. It indicates the time of day by the colour of its skin, which is dark by day and pale by night. The crabs changing colour follows a regular twenty-four hour plan that exactly matches the daily rhythm (节奏) of the sun.

    Does the crab actually keep time, or does its skin simply answer to the sun's rays, changing colour according to the amount of light strikes it? To find out, biologists kept crabs in a dark room for two months. Even without daylight, the crab's skin colour continued to change exactly on time.

    This characteristic(特性) probably developed gradually in answer to the daily rising and setting of the sun, to help protect the crab from sunlight and enemies. After millions of years it has become completely regulated (受控制) inside the living body of the crab.

    The biologists noticed that once each day the colour of the fiddler crab is especially dark, and that each day this happens fifty minutes later than on the day before. From this they discovered that each crab follows not only the rhythm of the sun but also that of the tides (潮水). The crabs period of greatest darkening is exactly the time of low tide on the beach where it was cought!

PRACTITIONERS

Jacqueline Felice de Almania(c.1322) highlights the suspicion that women practicing medicine faced.Born to a Jewish family in Florence,she moved to Paris where she worked as a physician and performed surgery.In 1322 she was tried for practicing unlawfully.In spite of the court hearing testimonials(证明)of her ability as a doctor,she was banned from medicine.

Tan Yunxian(1461-1554) was a Chinese physician who learned her skills from her grandparents.Chinese women at the time could not serve apprenticeships(学徒期) with doctors.However,Tan passed the official exam.Tan treated women from all walks of life.In 1511,Tan wrote a book,Sayings of a Female Doctor,describing her life as a physician.

James Barry(c.1789-1865) was born Margaret Bulkley in Ireland but,dressed as a man,she was accepted by Edinburgh University to study medicine.She qualified as a surgeon in 1813,and then joined the British Army,serving overseas.Barry retired in 1859,having practiced her entire medical profession living and working as a man.

Rebecca Lee Crumpler(1831-1895) worked as a nurse for eight years before studying in medical college in Boston in 1860.Four years later,she was the first African American woman to receive a medical degree.She moved to Virginia in 1865,where she provided medical care to freed slaves.