Hybrid cars are much more fuel efficient than gasoline powered cars, but they are still the most used cars today.

Who is "they" in this prior sentence?
gasoline powered cars
The army crossed the river because it was shallow.

Who is "it" in this prior sentence?
the river
DVD Players are slowly being replaced by Blu Ray Systems, but they will still exist for years to come.

Who is "they" in this prior sentence?
DVD Players