A compression spring is made of music wire of 20 mm diameter having a shear strength and shear modulus of 800 MPa and 80 GPa respectively. The mean coil diameter is 20 mm, free length is 40 mm and the number of active coils is 10. If the mean coil diameter is reduced to 10 mm, the stiffness of the spring is approximately
(a) Decreased by 8 times
(b) Deceased by 2 times
(c) Increased by 2 times
(d) Increased by 8 times