A Child Runs at 3

Multiple Choice

A child runs at 3.0 m/s and jumps onto a sled, initially at rest. If the child's mass is 36 kg, and if the child and sled slide off together at 2.0 m/s after the collision, the sled's mass is

A) 12 kg.
B) 18 kg.
C) 36 kg.
D) 48 kg.