Here's why:

* Terminal velocity is the constant speed that a freely falling object eventually reaches when the force of air resistance equals the force of gravity.

* Acceleration is the rate of change of velocity.

* When an object reaches terminal velocity, its velocity is no longer changing. Since acceleration is the change in velocity over time, and the velocity is constant at terminal velocity, the acceleration must be zero.

Think of it this way: Imagine you're skydiving. As you fall, you accelerate due to gravity. But as you speed up, the air resistance against your body increases. Eventually, the air resistance becomes equal to the force of gravity. At that point, you stop accelerating and reach terminal velocity.