<<The force of the
hand (which gave it the acceleration a) no longer acts on the ball.<<
I bet this is the problem. I think they think (and we haven't gotten to force yet) that the force is still with the ball even after it left the hand.
I know one of my students has answered along these lines.
Thanks
tina
Tina Fanetti
Physics Instructor
Western Iowa Technical Community College
4647 Stone Ave
Sioux City IA 51102
712-274-8733 ext 1429