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

在区间装、卸轨料及砂石料的作业，由（）担任施工负责人。

A.职务不低于班长的人员

B.职务不低于工长的人员

C.职务不低于车间主任的人员

D.段级领导