Are these all from No. 2 Middle School? -- Yes. They often their free time to help t

问题补充：

-- Are these all from No. 2 Middle School?

-- Yes. They often their free time to help the old here.

A. volunteers; volunteers

B. volunteer; volunteers

C. volunteers; volunteer

D. volunteer; volunteer

答案：

A. volunteers; volunteers

B. volunteer; volunteers

(答案→)C. volunteers; volunteer

D. volunteer; volunteer

解析：句意：这些志愿者都来自第二中学吗？是的。他们经常志愿把他们的空闲时间花在帮助这里的老人。前一空上的词是名词，后一空上的词是动词。所以选C。