If d is the thickness of a concrete pavement, the equivalent radius b of resisting section for an interior loading, is

• A. b = 0.6 + d2 - 0.675 d
• B. b = 1.6 + d2 + 0.675 d
• C. b = 1.6 + d2 - 3.675 d
• D. b = 1.6 + d2 - 0.675 d