A $3$ m high vertical earth retaining wall retains a dry granular backfill with angle of internal friction of $30^{\circ}$ and unit weight of $20 \: kN/m^3$. If the wall is prevented from yielding (no movement), the total horizontal thrust (in kN per unit length) on the wall is
- $0$
- $30$
- $45$
- $270$
