Question

Find the sum of the expression

$\frac{1}{\sqrt{1}+\sqrt{2}}+\frac{1}{\sqrt{2}+\sqrt{3}}+\frac{1}{\sqrt{3}+\sqrt{4}}+.........\frac{1}{\sqrt{80}+\sqrt{81}}$

      (a) 7                                                                    (b) 8

      (c) 9                                                                    (d) 10