单项选择题
A.pressure vacuum relief valves
B.excess flow valves
C.safety relief valves
D.flame screens
单项选择题 According to the regulations，normally，manholes in LFG tanks are located（）.
单项选择题 According to the Chemical Data Guide，when burning，the grade B flammable liquid，carbon disulfide produces（）.
单项选择题 According to regulations，a cargo hose used for transferring liquefied gases must have a bursting pressure of（）.