Two pieces of clay collide and stick together moving in one dimension. Clay 1 has a mass of 2,100 grams, and a clay 2 has a mass of 2,500 grams. Clay 1 has an initial velocity of 20 m/s immediately before they have a perfectly inelastic collision.

What is the final velocity of the two pieces of clay immediately after the collision?
a. 3.7 m/s
b. -3.7 m/s
c. 15 m/s
d.-15 m/s