Mario used to in the morning.A. practice swimmingB. practicing swimmingC. practice swimD.

问题补充：

Mario used to in the morning.

A. practice swimming

B. practicing swimming

C. practice swim

D. practices swimming

答案：

(答案→)A. practice swimming

B. practicing swimming

C. practice swim

D. practices swimming

解析：句意:马里奥过去常常在早晨联系游泳。本句中to是不定式符号，后面接动词原形，动名词短语作宾语表示抽象的动作，故选A。