If D and d are external and internal diameters of a circular shaft respectively, its polar moment of inertia, is

• A. <img src="/_files/images/civil-engineering/theory-of-structures/91-21-118-1.png">
• B. <img src="/_files/images/civil-engineering/theory-of-structures/91-21-118-2.png">
• C. <img src="/_files/images/civil-engineering/theory-of-structures/91-21-118-3.png">
• D. <img src="/_files/images/civil-engineering/theory-of-structures/91-21-118-4.png">