设 $\int f(x) \mathrm{d} x=F(x)+C$, 则 $\int \frac{1}{x^2} f\left(\frac{2}{x}\right) \mathrm{d} x=$.
$\text{A.}$ $F\left(\frac{2}{x}\right)+C$.
$\text{B.}$ $-F\left(\frac{2}{x}\right)+C$.
$\text{C.}$ $-\frac{1}{2} F\left(\frac{2}{x}\right)+C$
$\text{D.}$ $2 F\left(\frac{2}{x}\right)+C$.