Once people have seen something actually happens, they are much more to accept the change.
A. likely
B. possible
C. unlikely
D. probably
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(答案→)A. likely
B. possible
C. unlikely
D. probably
解析：句意：一旦人们看见过什么事情发生，他们更有可能接受改变，从句意排除C，unlikely“不可能”，D.probably也不对，A. likely和B. possible都是形容词“可能的”，但是likely可以用于Sb/sth is likely to do sth, possible用于：It is possible to do sth/ that句型，选A。