The kern of a circular cross-section of radius R is a concentric circular area with a radius of

• A. <img src="/_files/images/civil-engineering/upsc-civil-service-exam-questions/01-608-1.png">
• B. <img src="/_files/images/civil-engineering/upsc-civil-service-exam-questions/01-608-2.png">
• C. <img src="/_files/images/civil-engineering/upsc-civil-service-exam-questions/01-608-3.png">
• D. <img src="/_files/images/civil-engineering/upsc-civil-service-exam-questions/01-608-4.png">