Three CD's are bought at an average cost of $\$ 15$ each. If a fourth $\mathrm{CD}$ is purchased, the average cost becomes $\$ 16$. What is the cost of the fourth $\mathrm{CD}$ ?
(A) $\$ 16$
(B) $\$ 17$
(C) $\$ 18$
(D) $\$ 19$
(E) $\$ 20$