A propped cantilever of span $L$ carries a vertical concentrated load at the mid-span. If the plastic moment capacity of the section is $M_p$, the magnitude of the collapse load is

1. $\dfrac{8M_p}{L} \\$
2. $\dfrac{6M_p}{L} \\$
3. $\dfrac{4M_p}{L} \\$
4. $\dfrac{2M_p}{L}$