---What the forest of Yangzhou in the last 30 years?---Yangzhou has become much greener.A

问题补充：

---What the forest of Yangzhou in the last 30 years?

---Yangzhou has become much greener.

A. has happened to

B. was happened to

C. has happened at

D. is happening

答案：

(答案→)A. has happened to

B. was happened to

C. has happened at

D. is happening

解析：句意：在过去三十年当中，扬州的森林发生了什么？——扬州变得更加绿色了。短语happen to……发生了什么事。结合语境可知上文描述的是从过去一直持续到现在的动作，故用现在完成时态，选A。