hich of the following best describes how Einstein contributed to the development of the concept of gravity?
A
He described the movement of balls allowed to roll down inclined planes at various angles.

B
He said that all bodies would fall at the same speed if they were in a vacuum.

C
He said that the gravitational force exerted by an object is directly related to its mass.

D
He described gravity as curved space-time as opposed to a force.