A ball is thrown vertically upwards, reaches its highest point, then drops down to the ground, bounces once,reaches a height of 1.2 m, then falls again and finally rests on the grass. At which one or more times is the gravitational force on the ball zero?多项选择题

a. At the highest point it reached

b. At the instant the ball hit the ground for the first time

c. Whenever the ball was moving up

d. When the ball had bounced up to a height of 1.2 m

e. Before the ball was thrown

f. Whenever it was moving down

g. When the ball rested on the grass

h. The ball always experiences the force of gravity

i. At the exact instant the ball was thrown up