Ans: C
$$\begin{array}{|c|c|c|c|c|c|c|} \hline
& 1 & 2 & 3 & 4 & 5 & 6 \\ \hline
1 & 2 & 3 & 4 & 5 & 6 & 7 \\ \hline
2 & 3 & 4 & 5 & 6 & 7 & 8 \\ \hline
3 & 4 & 5 & 6 & 7 & 8 & 9 \\ \hline
4 & 5 & 6 & 7 & 8 & 9 & 10 \\ \hline
5 & 6 & 7 & 8 & 9 & 10 & 11 \\ \hline
6 & 7 & 8 & 9 & 10 & 11 & 12 \\ \hline
\end{array}$$
$$\begin{array}{|c|c|c|c|c|c|c|} \hline
& 1 & 2 & 3 & 4 & 5 & 6 \\ \hline
1 & 2 & 3 & 4 & 5 & 6 & 7 \\ \hline
2 & 3 & 4 & 5 & 6 & 7 & 8 \\ \hline
3 & 4 & 5 & 6 & 7 & 8 & 9 \\ \hline
4 & 5 & 6 & 7 & 8 & 9 & 10 \\ \hline
5 & 6 & 7 & 8 & 9 & 10 & 11 \\ \hline
6 & 7 & 8 & 9 & 10 & 11 & 12 \\ \hline
\end{array}$$
P(the sum of the two numbers thrown is a prime number)
$\begin{array}{cl}
= & \dfrac{15}{36} \\
= & \dfrac{5}{12}
\end{array}$
