The radiation view factor of the outer cylinder onto itself is

Question

Consider the radiation hear exchange inside an annulus between two very long concentric cylinders. The radius of the outer cylinder is R₀ and that of the inner cylinder is R_i. The radiation view factor of the outer cylinder onto itself is                                                                                                                                                     

      (a) $1-\sqrt{\frac{R_{i}}{R_{0}}}$                                                           (b) $\sqrt{1-\frac{R_{i}}{R_{0}}}$ 

      (c) $1-\left ( \frac{R_{i}}{R_{0}} \right )^{\frac{1}{3}}$                                                           (d) ${1-\frac{R_{i}}{R_{0}}}$