An object of mass $m$ travelling at velocity $v$ collides head-on with a stationary object of mass $3m$. If the two objects stick together, what fraction of the initial total kinetic energy is retained after the collision?

• A$\frac{1}{4}$
• B$\frac{1}{3}$
• C$\frac{1}{2}$
• D$\frac{3}{4}$

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