That day we ended up on the beach ________ aimlessly and the only sound was the waves ________ against the shore.

A．wandering; to beat

B．wandering; beating

C．to wander; beating

D．to wander; to beat

用一块长为a，宽为b(a＞b)的矩形木板，在二面角为α的墙角处围出一个直三棱柱的谷仓，试问应怎样围才能使谷仓的容积最大？并求出谷仓容积的最大值.