Si $y = ax^{n+1} + bx^{-n}$, entonces $x^2 \frac{d^2y}{dx^2}$ es igual a:

1. [a.] $n(n - 1)y$
2. [b.] $n(n + 1)y$
3. [c.] $ny$
4. [d.] $n^2y$