There is (There are) a high proportion of girls in their class.用那个对?

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There is (There are) a high proportion of girls in their class.用那个对?
a high proportion of girls 算是单数还是算复数,关系到用is还是用are.Steven said boys may do worse in English when there is a high proportion of girls in their class because they realize that the girls are better than them.此句出自浦东新区2012年中考预测英语试卷,或许用there are 才是最好的,但讲不清,

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