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

同时履行抗辩权在性质上属于( )。

A．法定权利

B．约定权利

C．延期抗辩权

D．永久抗辩权