Consider the power series $f(x) = \sum_{n=0}^{\infty} a_n x^n$ defined by the recurrence $a_0 = 1$, $a_1 = 0$, and $(n+2)(n+1)a_{n+2} + a_n = 0$ for $n \geq 0$. Which of the following functions does $f(x)$ represent?