In the final exam of last term, our maths teacher Mr.Wu gave us  ________we all couldn’t work out .

A．such a difficult problem that

B．so a difficult problem as

C．so difficult a problem that

D．as difficult a problem as

关于压力和压强，下列说法中正确的是（　　）

A．压力方向总与重力方向一致

B．压力越大，压强越大

C．压力一定时，受力面积越小，压强越大

D．压强是物体表面上受到的压力