[单选题]
A red ball with a vec r 2:m va9rcmc(k3(,qmg wr(wl-8,jji0q 3 smqeb f)ii1 se1eiocity of +3.0 m/s collides head-on with a yellow ball of equal mass movingf)e1se ieim 8qi j0 1q,b3s-ji with a velocity of -2.0 m/s. What is the velocity of the yellow ball after the collision?