A ball is tossed straight up from the surface of a small, spherical asteroid with no atmosphere. The ball rises to a height equal to the asteroid's radius and then falls straight down toward the surface of the asteroid. The acceleration of the ball at the top of its path is:

a. at its maximum value for the ball's flight.
b. equal to the acceleration at the surface of the asteroid.
c. equal to one-half the acceleration at the surface of the asteroid.
d. equal to one-fourth the acceleration at the surface of the asteroid.