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

A．to be hit

B．to have hit

C．to have been hit

D．to hit

使用过大彩色多普勒取样框会（）

A.降低血流成像的空间分辨率

B.降低血流成像的时间分辨率

C.提高检测血流速度

D.消除彩色信号的闪烁

E.增高帧频