正常人肌肉用力收缩时，众多运动单位电位相互重叠，以致不能分辨出基线，这种电位为（）

A.部分干扰电位

B.完全干扰电位

C.自发电位

D.束颤电位

E.病理性干扰电位

John passed the examination though he had only spent _______ on the subject as others had done.

A．as half time much

B．as half much time

C．half as time much

D．half as much time