—He looks unhappy.

—Yes, but he can't say what it is ______ is bothering him.[ ]

A. as          

B. that

C. what      

D. which

卵巢肿瘤最常见的并发症是

A．腹膜炎

B．感染

C．蒂扭转

D．破裂

E．恶变