The series of natural numbers is divided into groups
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The series of natural numbers is divided into groups as follows ; (1), (2, 3), (4, 5, 6),
(7, 8, 9, 10) and so on. The sum of the numbers in the nth group is(A) 1/2{n(n° +1)](B) n(n^2+1)/4(C) 2n(n + 1)/3(D) n^2(n+1) /2