Q 17

Q17. An isosceles trapezium ABCD is inscribed in a semicircle with centre 'O' and diameter AD as shown. If AD = 2a and AB = b then the length BC is equal to.

                             
    (A) (2a^​2 – b²) / a                           (B) 2 (a² – b²)/a                    (C) $\surd$(2a² – b²)                (D) 2​$\surd$(2a^{​ 2} – b²).