There are so many kinds of Mp4 in the shop. We can’t decide            .

A．to buy what

B．what to buy

C．to buy which

D．which to buy

如图所示，平行板电容器的电容为C，带电荷量为Q，板间距离为d，今在离右板d/4处放一电荷q，则它所受电场力的大小为 

A．

B．

C．

D．