A 2.0\,kg projectile is moving due east at 10\,m/s when it explodes into two fragments. The lighter fragment has mass 0.50\,kg and moves due north at 20\,m/s immediately after the explosion. Neglecting external forces, what is the speed of the heavier fragment?
a) 13.3m/s
b) 6.7m/s
c) 15m/s
d) 12m/s