The sum of the numerical coefficients in the expansion of the binomial $(a+b)^6$ is: 
$\textbf{(A)}\ 32 \qquad \textbf{(B)}\ 16 \qquad \textbf{(C)}\ 64 \qquad \textbf{(D)}\ 48 \qquad \textbf{(E)}\ 7$
$P(1,1)=(1+1)^6=2^6=64$, $\fbox{C}$