Question

didnt care what happened to himself, personally. he was expendable, and he knew it. perhaps he could forget about john for a while in there. he switched on his electric blower, and his suit puffed up around him. it didnt feel too bad, but it made him sweat profusely. the door was straight ahead. he held the map of the monkey house in his hand and nodded to captain haines. haines was ready. jerry opened the door, and they stepped inside. the sound of the monkeys became louder. they were standing in a windowless, lightless, cinderblock corridor that had doors at either end; this was the makeshift air lock, the gray zone. the rule inside the air lock was that the two doors, the far door and the near door, could never be open at the same time. this was to prevent a backflow of contaminated air from coming into the staging room. the door closed behind them, and the corridor went dark. it went pitch - dark.
what was one rule inside the air lock?
only use the electric blower when absolutely necessary.
never leave someone behind.
they needed to communicate using sign language.
the two doors could never be open at the same time.

Explanation:

The text clearly states "The rule inside the air lock was that the two doors, the far door and the near door, could never be open at the same time." This directly answers the question about the rule in the air - lock. The other options (using the electric blower, leaving someone behind, communicating via sign language) are not mentioned as rules for the air - lock in the given text.

Answer:

The two doors could never be open at the same time.