Earth is believed ______ by an object the size of Mars at some point in the distant past  

before its surface cooled．

A．to be hit

B．to have hit

C．to have been hit

D．to hit

下列症状中哪些是系统性硬皮病的症状（）

A.皮肤色素改变

B.雷诺现象

C.手弥漫性水肿

D.皮肤毛细血管扩张

E.结节性红斑