区队必须在作业规程中规定过断层至少（）棚的工字钢棚、背木、坑木、网片、防倒棚卡具、超前管缝等备用支护材料。

A、30

B、10

C、20

A flame screen（）.

A.permit the passage of vapor but not of flame

B.prevents the passage of flammable vapors

C.prevents inert gas from leaving a tank

D.permits vapors to exit but not enter a tank