Answer:
The 1st and 3rd answers are very good but I believe it's actually the 3rd, "Any noise might reveal that they are in the building.
Explanation:
They are trying to be quiet so that soldiers and other people don't hear them. I think they would try to be quiet so no soldiers here them but same thing for people.
Collective noun: several
Emphasis: multiple hours (<--- could be worded a different way if needed)