What is the approximate error in computed elongation?

Question

A bar of length L tapers uniformly from diameter 1.1 D at one end to 0.9 D at the other end. The elongation due to axial pull is computed using mean diameter D. What is the approximate error in computed elongation?                      

(a) 10%                 (b) 5%     (c) 1%        (d)0.5%