Only after he had spent several nights working on the math problem      to wonder whether the problem

was wrongly set.   [ ]

A. did he begin    

B. he began    

C. he did begin    

D. began he

甲、乙二物体都浸没在同种液体中，关于它们所受浮力大小的下面各种说法中正确的是（　　）

A．如果甲物体比乙物体重，则甲物体受到的浮力比乙物体受到的浮力大

B．如果甲物体的密度比乙物体大，则甲物体受到的浮力比乙物体受到的浮力小

C．如果甲物体的体积比乙物体小，则甲物体受到的浮力比乙物体受到的浮力小

D．如果甲物体空心，乙物体实心，则甲物体受到的浮力比乙物体受到的浮力大