The stiffness of the close coil helical spring is
A
$$\frac{{{{\text{d}}^4}{\text{N}}}}{{8{{\text{D}}^3}{\text{n}}}}$$
B
$$\frac{{{{\text{d}}^4}{\text{N}}}}{{4{{\text{D}}^3}{\text{n}}}}$$
C
$$\frac{{4{{\text{D}}^3}{\text{N}}}}{{{{\text{d}}^4}{\text{n}}}}$$
D
$$\frac{{8{{\text{D}}^3}{\text{N}}}}{{{{\text{d}}^4}{\text{n}}}}$$
Correct Answer: $$\frac{{{{\text{d}}^4}{\text{N}}}}{{8{{\text{D}}^3}{\text{n}}}}$$