农田生态系统经常采用______来加强对农副产品的转化和利用。
参考答案：食物链加环
— Li Na went out of the China Open in the first round. Why should she be defeated by such
an unknown player?
— Well, that's OK. She __________ enough after becoming the world champion. [ ]
A. could not train
B. must not have trained
C. can not train
D. may not have trained
