$x \rightarrow 0$ 时, 若 $\mathrm{e}^x-\frac{1+a x}{1+b x+c x^2}$ 是比 $x^3$ 高阶的无穷小量, 则
A. $a=\frac{1}{3}, b=-\frac{2}{3}, c=\frac{1}{6}$.
B. $a=\frac{1}{3}, b=-\frac{2}{3}, c=-\frac{1}{6}$.
C. $a=\frac{2}{3}, b=-\frac{1}{3}, c=-\frac{1}{6}$.
D. $a=\frac{4}{3}, b=\frac{1}{3}, c=\frac{1}{6}$.