a series of +名词+动词,动词用单数还是复数形式?由什么决定?

“a series of+名词”作主语时,动词用单数,如：A series of pre-recorded tapes has been prepared for language laboratory use。已经为语言实验室准备了一系列预录的磁带。同样的还有a kind of,a portion of等,这应该是固定用法 。