Two Blocks with Masses 2

Multiple Choice

Two blocks with masses 2.0 kg and 3.0 kg are placed on a horizontal frictionless surface. A light spring is placed in a horizontal position between the blocks. The blocks are pushed together, compressing the spring, and then released from rest. After contact with the spring ends, the 3.0-kg mass has a speed of 2.0 m/s. How much potential energy was stored in the spring when the blocks were released?

A) 15 J
B) 3.0 J
C) 6.0 J
D) 12 J
E) 9.0 J