Which one of the following accurately describes the force of gravity?

a. the force of gravity is slightly lower at the bottom of a mountain than it is at the top.
b. the gravity acceleration has no effect on a body moving in directly the opposite direction of the force of gravity.
c. the gravity acceleration is in the same direction as the force of gravity, and thus toward the center of the earth.
d. the gravity acceleration is in the same direction as the force of gravity, and thus away from the center of the earth.