A closely coiled helical spring of radius R, contains n turns and is subjected to an axial load W. If the radius of the coil wire is r and modulus of rigidity of the coil material is C, the stress developed in the helical spring is

• A. <img src="/_files/images/civil-engineering/strength-of-materials/140-6.212-1.png">
• B. <img src="/_files/images/civil-engineering/strength-of-materials/140-6.212-2.png">
• C. <img src="/_files/images/civil-engineering/strength-of-materials/140-6.212-3.png">
• D. <img src="/_files/images/civil-engineering/strength-of-materials/140-6.212-4.png">