问题
选择题
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
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。