it is said that there have been many changes ___in my hometown in the last ten years.

答案C是对的,因为前面已有动词have been
句型：THERE BE +名词+DOING STH ‘有……正在做……‘既然已经是there have been ，不可以是happened表示已经发生过了吗？不行记住：THERE BE +名词作主语（看到MANY CHANGES 了吗？），另外，HAPPEN 是不及物动词没有被动语态。