A Rock Is Dropped from Rest Off of a High

Multiple Choice

A rock is dropped from rest off of a high cliff on another planet, planet X. There is no atmosphere, and thus no air resistance, on planet X. At the end of 1 second, the rock is moving at a speed of 6 m/s. At the end of 2 seconds, it is moving at 12 m/s. How fast will the rock be moving at 4 seconds after being dropped?

A) 36 m/s
B) 24 m/s
C) 20 m/s
D) 40 m/s
E) 18 m/s