问题
单项选择题
0pen ullage holes in tanks which are not gas-free must be protected by（）.
A.PV valves
B.warning signs
C.flame screens
D.stop-check valves
答案
参考答案：C
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0pen ullage holes in tanks which are not gas-free must be protected by（）.
A.PV valves
B.warning signs
C.flame screens
D.stop-check valves
参考答案：C