Question on Summing up a series

Find the sum of the expression

(A) 7
(B) 8
(C) 9
(D) 10

Answer : (B)

Explanation : 

The expression can be written as :

= [sq_root(2) - sq_root(1) ] + [sq_root(3) - sq_root(2) ] + [sq_root(4) - sq_root(3) ] + ... + [sq_root(81) - sq_root(80) ]
= sq_root(81)  - - sq_root(1)
= 9 - 1
= 8

Hence the answer is 8.