[求助]一道语法题

kanstory

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.

正确答案是E. are caused by the interaction of two stars orbiting each other.

问题是 题目里的主语都是单数， ARE CAUSED是谁的谓语呀？？ 请教大家，多谢。

我是马甲哥哥

a number of astronomical phenomena

kanstory

问个弱弱的问题，A NUMBER OF 后面动词是单数还是复数。 我怎么感觉是加单数。多谢。