Two closely coiled helical springs with stiffnesses k1 and k2 respectively are connected in series. The stiffness of an equivalent spring is given by

• A. <img src="/_files/images/mechanical-engineering/machine-design/592-432-1.png">
• B. <img src="/_files/images/mechanical-engineering/machine-design/592-432-2.png">
• C. <img src="/_files/images/mechanical-engineering/machine-design/592-432-3.png">
• D. <img src="/_files/images/mechanical-engineering/machine-design/592-432-4.png">