Some are their time to help the old people.A. volunteer; volunteer B. volunteers; voluntee

问题补充：

Some are their time to help the old people.

A. volunteer; volunteer

B. volunteers; volunteering

C. volunteering; volunteers

D. volunteers; volunteers

答案：

A. volunteer; volunteer

(答案→)B. volunteers; volunteering

C. volunteering; volunteers

D. volunteers; volunteers

解析：句意：一些志愿者在自愿花时间帮助老人。volunteer志愿者，名词，volunteer to do sth.自愿去做某事，动词，这是一个现在进行时的句子，故选B。