The work of professional sectaries is so ______  that few people like to apply for it; it demands of not      only high working efficiency bust also ______  available all the time. As a result, they are ______ great    demand.      [ ]

A. demanding; being; in  

B. demanded; being; in  

C. demanding; be; in  

D. tiring; being; of

双集气管内的煤气压力应保持相等，防止煤气倒流。