It never humans until the mid1990s that some animals can be cloned.
A. occur
B. is occurred
C. occurred to
D. occurred
网友回答
A. occur
B. is occurred
(答案→)C. occurred to
D. occurred
解析：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。