Ans: C
Note that the sequence of the number of dots in the patterns is an arithmetic sequence. The common difference is $5$ and the first term is $9$. Therefore the number of dots in the $7$th pattern
Note that the sequence of the number of dots in the patterns is an arithmetic sequence. The common difference is $5$ and the first term is $9$. Therefore the number of dots in the $7$th pattern
$\begin{array}{cl}
= & 9 + (7-1) \times 5 \\
= & 39
\end{array}$
