Four combinations of two thin lenses are given in List I. The radius of curvature of all curved surfaces is r and the refractive index of all the lenses is 1.5. Match lens combinations in List I with their focal length in List II and select the correct answer using the code given below the lists.
 

	List I		List II
A.		$\mathrm{P}.$	$2\mathrm{r}$
B.		$\mathrm{Q}.$	$\frac{\mathrm{r}}{2}$
C.		$\mathrm{R}.$	$-\mathrm{r}$
D.		$\mathrm{S}.$	$\mathrm{r}$