已知地球的半径为R，质量为M，将地球看作均匀球体，若有可能将一质量为m的物体放在地球的球心处，则此物体受到地球的万有引力大小为（　　）

A．

B．无穷大

C．零

D．无法确定

Which should NOT be a treatment for a person who has received a head injury and is groggy or unconscious?（）

A.Give a stimulant

B.Elevate his head

C.Stop severe bleeding

D.Treat for shock