During a Snowball Fight Two Balls with Masses of 0

Multiple Choice

During a snowball fight two balls with masses of 0.6 and 0.4 kg,respectively,are thrown in such a manner that they meet head-on and combine to form a single mass.The magnitude of initial velocity for each is 15 m/s.What is the speed of the 1.0-kg mass immediately after collision?

A) zero
B) 3 m/s
C) 15 m/s
D) 6 m/s
E) ​9 m/s