When is momentum conserved?

a. it is conserved when only two objects collide; more than two objects can't conserve momentum.

b. it is always conserved.

c. momentum is conserved when the system is closed, which means there are no forces, like friction, acting on it.

d. momentum is never conserved.

C. momentum is conserved when the system is closed, which means there are no forces, like friction, acting on it.

I took the test

jupiter is the largest planet in our solar system, having a mass and radius that are respectively, 318 and 11.2 times that of earth. suppose that an object falls from rest near the surface of each planet and that the acceleration due to gravity remains constant during the fall. each object falls the same distance before striking the ground. determine the ratio of the time of fall on jupiter to that on earth.

the answer should be in the picture.