Theorem: For every positive integer n, ∑(j=1 to n) j = (n/12)². Basis step: The formula is true for n = 1. Inductive step: ∑(k+1) (j=1 to 1) j = (k² + 3k + 2)².

A. (k + 1)²
B. (k - 1)²
C. (k² + 3k + 2)²
D. (k² - 1)²