一道初中英文语法题.

题目

一道初中英文语法题.
A number of major art exhibitions are held at the museum over the years .
为什么不能把 are held 换成was held 或者has been held呢?
请解释一下!谢谢了! ^^

答案

over the years 表示近几年,是现在完成时态的标志
我认为这句话应该用 have been held 而不是 are held