In a reinforced concrete section, the stress at the extreme fibre in compression is $5.80$ MPa. The depth of neutral axis in the section is $58$ mm and the grade of concrete is $M25$. Assuming linear elastic behavior of the concrete, the effective curvature of the section (in per mm) is

1. $2.0 \times 10^{-6}$
2. $3.0 \times 10^{-6}$
3. $4.0 \times 10^{-6}$
4. $5.0 \times 10^{-6}$