A Brass Ball Is Shot Vertically Upward from the Surface

A brass ball is shot vertically upward from the surface of an atmosphere-free planet with an initial speed of 20.0 m/s. One second later, the ball has an instantaneous velocity in the upward direction of 15.0 m/s.
-What is the magnitude of the acceleration due to gravity on the surface of this planet?

A) 5.0 m/s²
B) 9.8 m/s²
C) 12 m/s²
D) 15 m/s²
E) 24 m/s²