谓语单复数的问题

作题目常常看到

a number of  astronomical phenomena are

an overwhelming proportion of women work

每次看到前面有 a 或是 an 的修饰都觉得很害怕

类似这样的用法应该要怎么判断

实在很困扰

求教了~

我只能说看看主语是什么

an overwhelming proportion of women work （是比例的话谓语随后一致）

the number of astronomical phenomena is  (主语是the number)

a number of  astronomical phenomena are  (主语是astronomical phenomena)