A ball, thrown vertically upward, is observed to move upward with a speed v1​ at time t1​ and a speed v2​, downward, at time t2​. The average velocity of the ball during the motion is (downward):
1. v2​−v1​​/2
2. v2​+v1​​/2
3. v2​−v1​
4. v2​+v1​