He is the only person who saw the accident ______. So he is the most important witness.[ ]

A. happen

B. to happen

C. having happened

D. happened

有关膀胱癌和结石的鉴别，下列错误的是()

A．双重对比造影两者等同

B．癌与结石均不随体位改变而移动

C．膀胱癌浸润膀胱壁，使膀胱壁不光整和硬化

D．膀胱癌的充盈缺损边缘不光整，而结石则光滑

E．X线平片易于诊断结石