AHSME 1983 Problem 30

Distinct points $A$ and $B$ are on a semicircle with diameter $MN$ and center $C$. The point $P$ is on $CN$ and $\angle CAP = \angle CBP = 10^{\circ}$. If $\stackrel{\frown}{MA} = 40^{\circ}$, then $\stackrel{\frown}{BN}$ equals

$\textbf{(A)}\ 10^{\circ} \textbf{(B)}\ 15^{\circ} \textbf{(C)}\ 20^{\circ} \textbf{(D)}\ 25^{\circ} \textbf{(E)}\ 30^{\circ}$