A geometric sequence has $a_2=12$ and $a_5=96$. Determine $a_1$ and $r$, and write the general formula for $a_n$.

$a_1=6$, $r=2$, $a_n=6\cdot 2^{n-1}$

$a_1=6$, $r=12$, $a_n=6\cdot {12}^{n-1}$

$a_1=12$, $r=2$, $a_n=6\cdot 2^{n-1}$

$a_1=4$, $r=3$, $a_n=4\cdot 3^{n-1}$