If the function $f(x) = \begin{cases} \frac{\sin 3x}{x}, & \quad x \neq 0\\ \frac{k}{2} & \quad x=0 \end{cases}$ continuous at $x=0$, then $k$ is equal to :

Question

( A ) 6
( B ) 9
( C ) 3
( D ) 12