单选题 We can learn from the passage that the tired land has fully recovered ______.

A、 when grasses and other plants appear again
B、 when the treatment is given by nature alone
C、 after it has been lying idle for ten years
D、 after grasses and other plants have decayed again

【正确答案】 D

【答案解析】根据文章第二段中的“The tired land then slowly recovered. Gradually grasses and other plants would appear on it and its productive power would be slowly by their decay .”则可知[D]正确。