估计样本含量的目的是在一定精度和检验效能的前提下确定（）

A.较多的观察单位数

B.最少的观察单位数

C.有效的观察单位数

D.随机的观察单位数

E.样本的观察单位数

When _____ why he went there,he said he was sent there to be trained for a space flight.  [ ]

A. asking                                      

B. being asked  

C. to ask                                      

D. asked