is a good form of exercise for both young and old.A. WalkB. WalkingC. The walkD. To walk

问题补充：

is a good form of exercise for both young and old.

A. Walk

B. Walking

C. The walk

D. To walk

答案：

A. Walk

(答案→)B. Walking

C. The walk

D. To walk

解析：考查非谓语动词。句意：散布是对于年轻人和老人来说锻炼的一种好形式，动名词作主语表示抽象动作，结合语境而可知选B。