A helical spring has N turns of coil diameter D and a second spring, made of same wire diameter and same material, has N/2 turns of coil of diameter 2D. If the stiffness of the first spring is k, then the stiffness of the second spring will be

A

$$\frac{k}{4}$$

B

$$\frac{k}{2}$$

C

2k

D

4k