A layer of normally consolidated, saturated silty clay of $1$ m thickness is subjected to one dimensional consolidation under a pressure increment of $20$ kPa. The properties of the soil are: specific gravity $=2.7$, natural moisture content $=45 \%$, compression index $=0.45$ and the recompression index $=0.05$. The initial average effective stress within the layer is $100$ kPa. Assuming Terzaghi’s theory to be applicable, the primary consolidation settlement (rounded off to the nearest mm) is
1. $2$ mm
2. $9$ mm
3. $14$ mm
4. $16$ mm