A two hinged parabolic arch of span l and rise h carries a load varying from zero at the left end to w per unit run at the right end. The horizontal thrust is

A

$$\frac{wl^{2}}{4h}$$

B

$$\frac{wl^{2}}{8h}$$

C

$$\frac{wl^{2}}{12h}$$

D

$$\frac{wl^{2}}{16h}$$