15. Among the following functions, which one is both an even function and monotonically decreasing on the interval $(0, +\infty)$? (A) $y = x^{-2}$ (B) $y = x^{-1}$ (C) $y = x^2$ (D) $y = x^{\frac{1}{3}}$
15. Among the following functions, which one is both an even function and monotonically decreasing on the interval $(0, +\infty)$?\\
(A) $y = x^{-2}$\\
(B) $y = x^{-1}$\\
(C) $y = x^2$\\
(D) $y = x^{\frac{1}{3}}$