[求助]Prep- P1- 136

136.
            (32028-!-item-!-188;#058&005735)

It seems likely that a number of astronomical phenomena, such as the formation of planetary nebulas, may be caused by the interaction where two stars orbit each other at close range.

(A) may be caused by the interaction where two stars orbit each other

(B) may be caused by the interaction between two stars that each orbit the other

(C) are because of the interaction between two stars that orbit each other

(D) are caused by the interaction of two stars where each is orbiting the other

(E) are caused by the interaction of two stars orbiting each other

答案选E，可是为什么是are呢？主语phenomena是单数啊。。。谁能帮我解释下？谢谢

a number of+n.动词用复数