Final answer:
The hot air balloon has zero acceleration because it is rising at a constant speed of 10 km/hr without any change in its velocity.
Step-by-step explanation:
The student is asking about the acceleration of a hot air balloon that is rising at a constant speed. If the speed of the hot air balloon is constant at 10 km/hr and there is no change in speed, its acceleration is zero. Acceleration is defined as the rate of change of velocity per unit time. Since there is no change in velocity (speed and direction) as the balloon is rising at 10 km/hr continuously without speeding up or slowing down, there is no acceleration in the balloon's movement.