若 $y = (1 + x^{2})^{2} - \sqrt{1 + x^{2}}$ , $y = (1 + x^{2})^{2} + \sqrt{1 + x^{2}}$ 是微分方程 $y' + p(x)y = q(x)$ 的两个解, 则 $q(x) = (\quad)$ (A) $3x(1 + x^{2})$ (B) $-3x(1 + x^{2})$ (C) $\frac{x}{1 + x^{2}}$ . (D) $-\frac{x}{1 + x^2}$ .