The stiffness of the close coil helical spring is_________________?
A. d4N/8D3n
B. d4N/4D3n
C. 4D3N/d4n
D. 8D3N/d4n
A. d4N/8D3n
B. d4N/4D3n
C. 4D3N/d4n
D. 8D3N/d4n