It never _____  humans until the mid­1990s that some animals can be cloned.
A．occurB．is occurredC．occurred toD．occurred
C

試題分析：It occurs to sb that 某人突然想到...，occur to 發(fā)生，突然想到。。。句意：直到20世紀(jì)中期人類才突然想到克隆出一些動(dòng)物。
點(diǎn)評(píng)：注意句型的識(shí)記。
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