县级行政区的设立、变更、撤销，由（）批准。

A.全国人民代表大会

B.全国人大常委会

C.国务院

D.省一级的人民政府

A virtual rise in the center of gravitty may be caused by（）.

A.filling a partially tank

B.using an on board crane to life a freely swinging heavy object

C.emptying a partially filled tank

D.transferring ballast from the forepeak to the after peak