寒冷地区某地下室长100m、宽90m，水土无侵蚀性，仅考虑混凝土收缩问题，地下室外墙混凝土强度等级适宜的是：（）。

A.C30 

B.C35 

C.C40 

D.C50

—The woman biologist stayed in Africa studying wild animals for 13 years before she returned.

—Oh, dear! She _______ a lot of difficulties!

A．may go through

B．might go through

C．ought to have gone through

D．must have gone through