The sum of the first n terms of a linear sequence is (S_{n} = n^{2} + 2n). Find the common difference of the sequence.

A.
5 
B.
4 
C.
3 
D.
2
Correct Answer: Option D
Explanation
(S_{n} = frac{n}{2}(2a + (n – 1)d )
(frac{n}{2}(2a + (n – 1) d = n^{2} + 2n)
(n(2a + (n – 1) d = 2n^{2} + 4n)
(2an + n^{2}d – nd = 2n^{2} + 4n)
Comparing the equations, d = 2 (coefficient of (n^{2})).