当 $x \to 0$ 时, $f(x) = x - \sin ax$ 与 $g(x) = x^2 \ln (1 - bx)$ 是等价无穷小量, 则( ) (A) $a = 1, b = -\frac{1}{6}$ . (B) $a = 1, b = \frac{1}{6}$ . (C) $a = -1, b = -\frac{1}{6}$ . (D) $a = -1, b = \frac{1}{6}$ .