Question

It never _____  humans until the mid­1990s that some animals can be cloned.

A．occur

B．is occurred

C．occurred to

D．occurred

Answer 1

C

试题分析：It occurs to sb that 某人突然想到...，occur to 发生，突然想到。。。句意：直到20世纪中期人类才突然想到克隆出一些动物。
点评：注意句型的识记。