---There are many volunteers are helping the children in Sichuan.And most of them are

问题补充：

---There are many volunteers are helping the children in Sichuan.

--- And most of them are college students.

A. which

B. when

C. whose

D. who

答案：

A. which

B. when

C. whose

(答案→)D. who

解析：句意：在四川有许多帮助孩子们的志愿者。——他们中大部分是大学生。上文中先行词volunteers为下文宾语从句中的逻辑主语，指人，故引导词用who，选D。