She _____me only twice since last year.   [ ]

A. sees              

B. was seeing        

C. has seen          

D. have seen

采用正态近似法估计总体率的置信区间，要求（）

A.n≥50

B.p不接近0

C.p接近0.50

D.np或n（1-p）大于5

E.np与n（1-p）均大于5