Which revision correctly uses an appositive phrase to combine these two sentences? My brother's prized possession is a small toy ambulance. He often keeps it in his pocket.

My brother often keeps his prized possession, a small toy ambulance, in his pocket.

My brother often keeps a small toy ambulance in his pocket, which is his prized possession.

My brother's prized possession is a small toy ambulance, and he often keeps it in his pocket.

My brother often keeps a small toy ambulance in his pocket because it is his prized possession.