问题描述：
这个句子为何用ING形式?
1.Duties like organizing dinners for world leaders and politicians
2.Missus Clinton has been campaigning in New York for months
还有3.Or welcoming groups to the White House
1.Duties like organizing dinners for world leaders and politicians
2.Missus Clinton has been campaigning in New York for months
还有3.Or welcoming groups to the White House
问题解答：
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