He ________ get into his own house because he has lost his keys.

A．mustn’t

B．doesn’t have to

C．can’t

D．needn’t

根据以往调查的资料，某城市职工平均每户拥有国库券和国债的方差为1600，为使极限抽样误差在概率保证程度为0.9545时不超过4元，应抽取（）户来进行调查。

A.I600

B.400

C.10

D.200