--The world's population is getting larger and larger. It's a serious problem.

--Yes. Scientists will have to        new ways        food problem.

A. think of, to solve

B. think over, of solve

C. think about, about solving

机车乘务组以外人员登乘机车时，除铁道部机车运用规程指定的人员外，须凭（）登乘。

A、调度命令

B、工作证

C、登乘机车证

D、司机保单