在没有氧气的条件下，一些细菌将秸秆、粪便、污水中的有机物通过发酵分解产生一种可用于照明、取暖、做饭的气体，产生的这种气体可能是[ ]

A．一氧化碳    

B．沼气  

C．氧气    

D．二氧化碳

The existing terrestrial radio system has a number of basic flaws which（）.

A.can be overcome by using traditional techniques

B.cannot be overcome by satellite system

C.can be overcome by satellite and digital techniques

D.cannot be tackled by any modern system