Sharon kicks a ball from the ground into the air with an upward velocity of 64 feet per second. The function h = -16t2 + 64t models the height h, in feet, of the ball at time t, in seconds. When will the ball reach the ground again?

a. 1 second after the ball is thrown
b. 2 seconds after the ball is thrown
c. 3 seconds after the ball is thrown
d. 4 seconds after the ball is thrown