A ball of mass $m$ moving at a speed $v$ makes a head-on collision with an identical ball at rest. The kinetic energy of the balls after the collision is three fourth of the original. The coefficient of restitution is:
1. $\frac{1}{2}$
2. $\frac{1}{3}$
3. $\frac{1}{\sqrt{2}}$
4. $\frac{1}{\sqrt{3}}$
