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。