--What on earth are the police busy with?

---They have been looking into how the plane crash _______.

A．came out

B．came up

C．came down

D．came about

安全生产完成任务最基本条件是什么？（）

A、设备运行正常

B、性能完好

C、机件无损、机件整洁

D、零附件齐全好用