—It’s reported that the new underground line has been completed．

—Yes but it hasn’t been made clear _____it’s to be opened to traffic．

A．that

B．who

C．what

D．when

在污染跑道上着陆（）

A.应避免在没有防滞的情况下着陆

B.如果没有防滞只要性能允许就应落地

C.按特殊的程序做没有污染的跑道上着陆