Think again before you choose!

$\{a_n\}$ is an arithmetic sequence , $\{b_n\}$ is a sequence such that $b_{n}=a_{n} a_{n+1} a_{n+2}$ for positive integer $n \geq 1$ .

Let $S_n$ denote the sum of the first $n$ terms of $\{b_n\}$ .

If $4a_{5}=7a_{12}>0$ , for which value of $n$ will $S_n$ reach the maximum ?