Two different soil types (Soil $1$ an Soil $2$) are used as backfill behind a retaining wall as shown in the figure, where $\gamma_t$ is total unit weight, and $c’$ and $\phi’$ are effective cohesion and effective angle of shearing resistance. The resultant active earth forceper unit length (in kN/m) acting on the wall is

1. $31.7$
2. $35.2$
3. $51.8$
4. $57.0$