There might be life on other planets, but so far scientists haven't found enough ________ to prove it.

A．evidence　　

B．relic

C．mystery

D．reception

引起老年人胸痛的常见原因是（）

A.胸膜炎

B.肺炎

C.心绞痛

D.风湿性心脏病

E.自发性气胸