问题描述：
-ed与-ing的区别
比如：There are students and people from all walks of life _____（volunteer）to work in the EXPO site.
答案是volunteering,为什么不是-ed呢,区别是什么呢
比如：There are students and people from all walks of life _____（volunteer）to work in the EXPO site.
答案是volunteering,为什么不是-ed呢,区别是什么呢
问题解答：
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