Why are force fields necessary to describe gravity?

a. gravity can only push objects apart
b. gravity is a type of air resistances
c. gravity acts between any two objects with charges
d. gravity is a non-contact force

D. gravity is a non-contact force      

Explanation:

Gravitational force acts between two bodies having mass. The force is attractive in nature and is independent of the mass of the bodies. Thus, gravity can pull objects towards each other.

The magnitude of this force depends on the mass of the two bodies and inversely proportional to the square of the distance between them.

It is a non-contact force. So, to describe gravity, force fields are necessary. Force fields are used to describe non-contact forces- force on any body at any point in space.

Thus, correct option is D.  

t = 1/400 h   ⇔ 0.0025 h ⇔ 9s ⇔ 0.15 m

any of the answers are correct.