单选题 Which is the main idea of the first paragraph?

A、 Scientists know how to change desert into good land.
B、 Land is becoming desert faster than scientists can change it back into good land.
C、 If scientists can bring water to desert, people can live and grow food theory.
D、 More and more places are becoming deserts all the time.

【正确答案】 B

【答案解析】第一段主要讲了两个方面，一方面科学家努力要把沙漠改造成良田，另一方面，人们都在违背科学行事，所以土地沙化的速度大大超过科学家改良的速度，因此，B为正确答案。