题目内容：
根据要求，回答题：
A．formed over many centuries from plants.
B．returns it to the earth in the form of rain or snow.
C．are being used up very quickly.
D．there would be no life.
E．produce mechanical energy and electricity.
F．are being produced everyday.
请在（46）处填入最佳答案
参考答案：
答案解析：