The split between humans and apes is believed by scientists 15 to 20 million years ago.
A. to be occurring
B. having occurred
C. to have occurred
D. to occur
网友回答
A. to be occurring
B. having occurred
(答案→)C. to have occurred
D. to occur
解析：句意：科学家们认为人类和猿类的分化发生在1500百万到2000百万年前。不定式的动作的发生要早于谓语动词动作的发生，因此用不定式的完成式，选C。