预加氢反应是体积减小的反应，因此增大反应压力，有利于脱去油品中的杂质，所以在设备允许的条件下，尽量将反应压力控高些。()

A. having
B. had
C. having been
D. have

A. in the development of air transportation
B. the earliest passenger flights were successfully operated
C. to make travel easy and pleasant for the passengers
D. to provide different services
E. the shortage of qualified pilots
F. traveling by air was very cheap

A. as many as
B. as well as
C. as soon as
D. as much as