(NEED HELP OR I WILL FAIL). Why is the observed distance of the moving ball less when the observer is on the truck?

An upwardly moving ball which is slowing down is said to be undergoing a downward acceleration. As the ball falls, it also undergoes a downward acceleration. A downwardly moving ball which is gaining speed is said to have a downward acceleration. This downward acceleration is attributed to the downward force of gravity which acts upon the ball. If the ball's motion can be approximated as projectile motion (that is, if the influence of air resistance can be assumed negligible), then there will be no horizontal acceleration. In the absence of horizontal forces, there would be a constant velocity in the horizontal direction. This explains why the ball would be located directly above the launcher from which it is projected.

Many would insist that there is a horizontal force acting upon the ball since it has a horizontal motion. This is simply not the case. The horizontal motion of the ball is the result of its own inertia. When projected from the truck, the ball already possessed a horizontal motion, and thus will maintain this state of horizontal motion unless acted upon by a horizontal force. An object in motion will continue in motion with the same speed and in the same direction ...

a and c

c is 100% but i am confused in a

frequency or wavelenth of em radiation in a vacuum