Answer:
The force acting on the ball is 92.4 N.
Explanation:
Given that,
Initial speed of the ball, u = 0
Final speed of the ball, v = 31.6 m/s
The average power generated during the serve is 2920 W. Power generated by an object is given by :

W is the work done, W = Fd

Since, 
So,

F is the force acting on the ball

So, the force acting on the ball is 92.4 N. Hence, this is the required solution.
I would expect it to be slightly basic.
Choices 'C' and 'D' are both correct.
(Except in 'C', changing the temperature from 1°C to 3°C is not usually
described as 'cooling', and it's not the water's 'mass' that changes. But
water does contract in volume during that change.)
Answer:
It conserves both energy and momentum in the collision at the same time. By design, when the balls collide the strings that hold them up are vertical (assuming balls are only swung from one side).
Explanation:
Hope This Helps!!