Volunteering our time to help people a good way to our free time.A. be; spendB. are; cost

问题补充：

Volunteering our time to help people a good way to our free time.

A. be; spend

B. are; cost

C. is; spend

D. will; cost

答案：

A. be; spend

B. are; cost

(答案→)C. is; spend

D. will; cost

解析：句意：花时间志愿帮助别人是我们花费空闲时间的好方法。Volunteering our time to help people是动名词做主语，说的是花时间志愿帮助别人这件事情，是第三人称单数，要用is,spend花费，人作主语，cost花费，物作主语，结合语境可知选C。