56. The shopkeeper _____ the tramp(流浪汉) stealing sweets from the new supermarket and _____ him.

A．catch; seize

B．caught; seized

C．seized; caught

D．caught; caught

关于线性规划问题，叙述正确的为（）。

A.其可行解一定存在

B.其最优解一定存在

C.其可行解必是最优解

D.其最优解若存在，在可行解中必有最优解