The Backstreet Boys were much more popular than N-Sync since they had the better music.

Who is "they" in this prior sentence?
The Backstreet Boys
The cap fell off the bottle because it was pressurized.

Who is "it" in this prior sentence?
the bottle
The Drill Sergeants trained the troops aggressively because they need to be trained well to survive in war.

Who is "they" in this prior sentence?

the troops