Question

It is a speed time graph for the falling ball until it hits the ground

what does this mean
"the acceleration of the ball decreases" ?

Answer 1

Theball motion by gravity (the free slope) is equally accelerated.
Acceleration is constant (until it hits the ground).
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Generallyunder a drawing of dependence of velocity it is easy to spot as acceleration
changes, if aspeed time graph is a straight line – acceleration constant.