根据短文内容，从题中所给的A、B、C、D四个选项中选出最佳答案。

     If you like to check your smartphone from time to time, you are not alone. In fact, using apps（应用程序）has now become part of everyday life.

Changba

      Changba, which has been described as "the best mobile karaoke for sound", allows you to sing along to over 2,000 songs. You can also share your KTV recordings with friends on social media sites such as Weibo, Wechat, Qzone, etc. The app allows users to create a photo show to go with their songs and comment on other people's songs. Changba can even filter the sound. For example, it can make cat noises sound like a beautiful chorus(合唱).

Moment Cam

     Moment Cam brings you some of the most useful features a camera app can have, such as adjusting the flash(调整闪光灯）,continuous shots, adjusting the timer, and choosing different sizes for photo-sharing sites. But what sets the app apart from all the others is the choice of simple mini-buttons that you hardly notice are there.

Carrot

     Carrot, a to-do list app, is a task manager with a personality, When you succeed in finishing your tasks, for example, Carrot praises you with a message such as "Way to go. Soon you'll be completing other people's to-do lists".But if you fail to perform your tasks, Carrot might say, "I'm angered at your behavior." And it may change its background from white to black to show how angry it is. So, if you need an extra push to finish your tasks, give Carrot a try.

    In many high schools, there are strict rules about using mobile phones. If your phone rings in class, for example, the teacher will be very angry.

    Every mobile phone has a ringtone (铃声). It tells you that someone is calling you. Now, there is a ringtone called “mosquitotone (蚊子铃声)”.

    Mosquitotone has a special sound. Only children and teenagers can hear it. As we get older, we are unable to hear some sounds with high frequencies (频率). As a result, a child or teenager can hear the mosquitotone ringtone, but someone of 20 or older can't.

    But sometimes, people over 20 can hear the ringtone. One British student's phone uses mosquitotone. In class, the student's phone rang. The 22-year-old teacher looked at the student and became unhappy. “I heard that. You know the rule：no mobile phone in class. ”

    Not all the students are interested in mosquitotone. Jerry Jones, a high school student of 17, says, “Using mosquitotone too often is bad for our hearing. I may use it for pleasure, but not in class. It's unfair(不公平的) to the teachers！”

    What do parents think of mosquitotone？“I think it is perfect for places like movie theaters,” says Connie Neeson, 44. “Think about it. You're watching a film and someone's mobile phone rings. Mosquitotone is good because many people don't hear it. However, it also has a bad influence（影响） on their study if students use it in class. ”

    Is there intelligent(有智力的) life on other planets(行星)? For years, scientists said "No." or "We don't know." But today this is changing. Seth Shostak and Alexandra Barnett are astronomers (天文学家). They believe intelligent life exists somewhere in the universe. They also think we will soon contact these beings(人；生物).

    Why do Shostak and Barnett think intelligent life exists on other planets? The first reason is time. Scientists believe the universe is about 12 billion years old. "This is long enough for other planets to have intelligent life," say Shostak and Barnett. The second reason is size——the universe is huge. "Tools like the Hubble Telescope (哈勃望远镜) have shown that there are at least 100 billion galaxies (银河系) ," says Shostak. "And our galaxy, the Milky Way, has at least 100 billion stars. Some planets going around these stars might be similar to Earth. "

    In the past, it was hard to look for signs of intelligent life in the universe. But now, powerful telescopes (高倍望远镜) allow scientists to discover smaller planets--the size of Mars or Earth--in other solar systems (太阳系).These planets might have intelligent life.

    Have beings from space already visited Earth? "Probably not," says Shostak. "It's a long way away. However, intelligent beings may contact us in other way, such as radio signals(信号). "In fact, they may be trying to communicate with us now, but we don't have the right tools to receive their messages. However, this is changing. By 2025, we could make contact with other life forms in our universe and we might help each other.

    Can trees talk? Yes, but not in words. Scientists have reasons to believe that trees do communicate with each other. Not long ago, researchers learned some surprising things. Some caterpillars(毛毛虫) attacked a willow tree(柳树) in the woods. The tree changed the chemistry of its leaves and the leaves tasted so terrible that the caterpillars stopped eating them. Then even more surprising, the tree sent out a special vapour (蒸气) a signal causing its neighbours to change the chemistry of their own leaves and make them less tasty.

    Communication, of course, doesn't need to be always in words. We can talk to each other by smiling, raising our shoulders and moving our hands. We know that birds and other animals use a whole vocabulary of songs, sounds and movements. Bees dance their signals, flying in certain patterns that tell other bees where to find nectar(花蜜) for honey. So why shouldn't trees have ways of sending messages?