阅读短文，从每题所给的A、B、C和D四个选项中，选出最佳选项。
Dujiangyan is the oldest manmade water
system in the world, and a wonder in the development of Chinese science. Built
over 2, 200 years ago in what is now Sichuan Province in Southwest China, this
amazing engineering achievement is still used today to irrigate over 6, 000
square kilometers of farmland, take away floodwater and provide water for 50
cities in the province.
In ancient times, the region in which
Dujiangyan now stands suffered from regular floods caused by overflow from the
Minjiang River. To help the victims of the flooding, Li Bing, the region's
governor, together with his son, decided to find a solution. They studied the
problem and discovered that the river most often overflowed when winter snow at
the top of the nearby Mount Yulei began to melt as the weather warmed.
The simplest fix was to build a dam, but
this would have ruined the Minjiang River. So instead Li designed a series of
channels built at different levels along Mount Yulei that would take away the
floodwater while leaving the river flowing naturally. Better still, the extra
water could be directed to the dry Chengdu Plain, making it suitable for
farming.
Cutting the channels through the hard rock
of Mount Yulei was a remarkable accomplishment as it was done long before the
invention of gunpowder and explosives. Li Bing found another solution. He used
a combination of fire and water to heat and cool the rocks until they cracked
and could be removed. After eight years of work, the 20metrewide canals had
been carved through the mountain.
Once the system was finished, no more floods
occurred and the people were able to live peacefully and affluently. Today,
Dujiangyan is admired by scientists from around the world because of one
feature. Unlike modern dams where the water is blocked with a huge wall,
Dujiangyan still lets water flow through the Minjiang River naturally, enabling
ecosystems and fish populations to exist in harmony.
