$\begin{array}{1 1} (a) \;T= \sqrt {\large\frac{dr}{s}} \\ (b) \;T= \sqrt {\large\frac{r^3 d}{s}} \\ (c) \;T= \sqrt {\large\frac{s}{dr}} \\ (d) \;T= \sqrt {\large\frac{r^2d}{s^2}} \end{array} $