必须是
be doing: 代表了正在干某事现在进行时
I'm doing my home work now
doing：动词ing形式做名词用
swimming is good for our health.
I like swimming
当动词be后所接的名词是单数可数名词或不可数名词时，be 应该取单数is；当其后所接的名词是复数的可数名词时，be用复数are。
例： There is/are sb doing…
There is a bird flying in the sky.
There are two boys swimming across the river.
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