问题
单项选择题
Over a very large number of trials, the probability of an event ______ is equal to the probability that it will not occur.
A．occurring
B．occurred
C．occurs
D．occur
答案
参考答案：A
解析：[分析] 固定搭配。 分词作定语修饰event，由于表主动，所以用现在分词。