A cantilever beam of length $2$ m with a square section of side length $0.1$ m is loaded vertically at the free end. The vertical displacement at the free end is $5$ mm. The beam is made of steel with young's modulus of $2.0 \times 10^{11} N/m^2$. The maximum bending stress at the fixed end of the cantilever is

1. $20.0$ MPa
2. $37.5$ MPa
3. $60.0$ MPa
4. $75.0$ MPa