If `alpha` an integer and `P (alpha, alpha^2)` is a point in the interior of the quadrilateral `x=0, y=0, 4x+y-21=0 and 3x+y-4=0, and (1+ax)^n = 1 + 8x+24x^2 + …,` then `alpha=` (A) `a` (B) `-a` (C) `a^2` (D) none of these