Conservation of momentum for collision(q59)

[jack1234]

For the following problem:
http://tinyurl.com/28xuwh

The answer is a.
What I understand is: The momentum of a system of two particles is conserved when the system is isolated.

Can anybody explain to me, why a is the correct answer?

 

[neutrino]

One way of finding out why option a is true, or why the others aren't, is to see what b,c,d and e would lead to, assuming they are correct.

Remember, Force = dp/dt.