If $\large\frac{a}{b}$ and $\large\frac{c}{d}$ are two rational such that $\large\frac{c}{d} $$\neq 0 $ then $\large\frac{a}{b} \div \frac{c}{d}$=________

Question

$\begin{array}{1 1} \large\frac{a}{b}+ \frac{c}{d} \\\large\frac{a}{b} \times \frac{c}{d}\\\large\frac{a}{b} \times \frac{d}{c}\\\large\frac{b}{a}+ \frac{d}{c} \end{array} $