the split between humans and living ages is thought by some scholars ____15 to 20 million years ago

题目

the split between humans and living ages is thought by some scholars ____15 to 20 million years ago
为什么填 to have occurred

答案

be thought to do 被认为做某事
因为后边是 15 to 20 million years ago 已经发生了!
因此用 不定式的完成式
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