If R is the radius of a main circular curve, Δ is the angle of deflection and α is the polar deflection angle of any point, its radial distance is

• A. 3 R sin Δ/3 sin 2α
• B. 3 R sin Δ/2 sin 3α
• C. 3 R sin Δ/3 sin α/2
• D. 3 R sin 2Δ sin α