The split between humans and living apes is thought by some scholars 15 to 20 million years ago.
A. to occur
B. occurring
C. to have occurred
D. having occurred
网友回答
A. to occur
B. occurring
(答案→)C. to have occurred
D. having occurred
解析：本题考查非谓语动词。 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。