问题
选择题
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
答案
答案：C
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。