Water and gas lines should have shutoff values and positive drains. What is a positive drain?

A.
Water does not go into the drain until a fire has been detected.

B.
This characteristic ensures that the pipe is unbreakable.

C.
Water, or gas, flows out instead of in.

D.
Water, or gas, flows in instead of out.

Explanation:
Positive drain means that contents flow out instead of in. If the
surrounding area flooded for some reason, a company wants to ensure that outside
water and other substances are not allowed to flow back into their facility through
pipes.