At the same instant that a 0.50-kg ball is dropped from 25 m above Earth, a second ball, with a mass of 0.25 kg, is thrown straight upward from Earth's surface with an initial speed of 15 m/s. They move along nearby lines and pass without colliding. At the end of 2.0 s the velocity of the center of mass of the two-ball system is:
a) 11 m/s, down
b) 15 m/s, up
c)20 m/s, down
d)11 m/s, up
e)15 m/s, down