c
${C_2} \to {C_2} + {C_1}$
$ \Rightarrow \left| {\begin{array}{*{20}{c}}
{{\,^9}{C_4}}&{{\,^{10}}{C_5}}&{{\,^{10}}{C_r}}\\
{{\,^{10}}{C_6}}&{{\,^{11}}{C_7}}&{{\,^{11}}{C_{r + 2}}}\\
{{\,^{11}}{C_8}}&{{\,^{12}}{C_9}}&{{\,^{12}}{C_{r + 1}}}
\end{array}} \right| = 0$
$ \Rightarrow \boxed{r = 5}$