A soldier loads a 10 $\mathrm{kg}$ cannonball into a 300 $\mathrm{kg}$ cannon that is initially at rest on the ground. What is the recoil speed of the cannon if the cannonball is fired with a horizontal velocity of 300 $\mathrm{m} / \mathrm{s}$ ?
(A) 5 $\mathrm{m} / \mathrm{s}$
(B) 7.5 $\mathrm{m} / \mathrm{s}$
(C) 10 $\mathrm{m} / \mathrm{s}$
(D) 15 $\mathrm{m} / \mathrm{s}$