A retaining wall of height $H$ with smooth vertical backface supports a backfill inclined at an angle $\beta$ with the horizontal. The backfill consists of cohesionless soil having angle of internal friction $\phi$. If the active lateral thrust acting on the wall is $P_a$, which one of the following statements is TRUE?

- $P_a$ acts at a height $H/2$ from the base of the wall and at an angle $\beta$ with the horizontal
- $P_a$ acts at a height $H/2$ from the base of the wall and at an angle $\phi$ with the horizontal
- $P_a$ acts at a height $H/3$ from the base of the wall and at an angle $\beta$ with the horizontal
- $P_a$ acts at a height $H/3$ from the base of the wall and at an angle $\phi$ with the horizontal