在PCR反应中，经过n次循环，理论上DNA链的数目扩增了多少倍（）

A.n倍ⁿ

B.2倍ⁿ

C.4倍ⁿ

D.8倍ⁿ

E.16倍ⁿ

— 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