When proving by the weak form of the Principle of Mathematical induction that, "for all positive integers n, the sum of the first n positive integers is n( n+1)/2," what would the inductive hypothesis be?
A. Assume 1+2+…+k=k(k+1)/2 holds for some positive integer k.
B. Assume 1=1(1+1)/2 holds. Assume 1+2=2(2+1)/2 holds.
C. Assume k(k+1)/2+(k+1)=(k+1)(k+2)/2 holds for some positive integer k.
D. Assume 1+2+…+k+(k+1)=(k+1)(k+2)/2 holds for some positive integer k.