问题
选择题
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
答案
答案：C
题目分析：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。