How do you think            that he was an hour late on such a short journey?

A．it came about

B．did it come about

C．it was come about

D．was it come about

PSA吸附塔，床层压力变化过快的处理方法有（）

A、调慢程控阀关阀速度

B、调快程控阀开阀速度

C、调慢程控阀开阀速度

D、调快压控阀开阀速度