The speed-density relationship in a mid-block section of a highway follows the Greenshield’s model. If the free flow speed is $v_f$ and the jam density is $k_j$, the maximum flow observed on this section is

1. $v_fk_j \\$
2. $\dfrac{v_fk_j}{2} \\$
3. $\dfrac{v_fk_j}{4} \\$
4. $\dfrac{v_fk_j}{8}$