Answer:
The correct answe is: because of the gravity and oxygen.
Step-by-step explanation:
Although planets such as Mars and Venus have a small atmosphere, they do not have the large amount of oxygen our planet has and cannot harbor life because of this reason. The atmosphere protects us from sunlight and is what allows life to exist.
The planet already had an atmosphere, but it was not strong enough, and as the planet cooled and the gases appeared, our atmosphere became stronger.