A. Go left
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You go left, and after some uncomfortable crawling, you see some light coming through a grate in the bottom of the vent, peeking in, you can see it's the locker room for the troops, and there's some unequipped guys down there, about three from what you can see through the grate.
A.) Do you leap down and attack quickly
B.) Do you wait for them to leave, then sneak down.
C.) Open the grate, and attempt to sneak by.
D.) make a loud noise... by rolling a grenade out of the vent, on top of the troops. (Makes the rest of the infiltration much harder but actiony)