More bison were killedThe bison population is thought from 60 million to just a few hund

问题补充：

More bison were killed．The bison population is thought from 60 million to just a few hundred between 1850 and 1910.

A. to have fallen

B. having fallen

C. to fall

D. falling

答案：

(答案→)A. to have fallen

B. having fallen

C. to fall

D. falling

解析：考查句型;Sb /sth is thought to have done，某人据认为已经做了什么，句意是;更多的野牛被杀死，野牛数量被认为在1850到1910之间已经从6千万下降到几百头了。选A。