Life is like riding a bicycleyour balance you must keep moving.A. KeepingB. To keepC.

问题补充：

Life is like riding a bicycle. your balance, you must keep moving.

A. Keeping

B. To keep

C. Keep

D. Having kept

答案：

A. Keeping

(答案→)B. To keep

C. Keep

D. Having kept

解析：B 考察非谓语动词。句义：生活就像骑自行车，为了保持平衡，你必须不断前进。根据句义可知此处应该使用不定式表示目的。故B正确。