It's difficult for me to decide which one to choose, because ________ of them are good.  [ ]

A. neither              

B. both            

C. either            

D. each

对直升机而言地面效应的产生对其悬停产生何种作用（）

A.不利于直升机的悬停

B.容易造成力的不平衡

C.升力增大有利于悬停

D.无影响