A $C_{n}^{l}C_{l}^{r}=C_{n}^{r}C_{l-r}^{n-r}$

B $C_{k}^{k}+C_{k}^{k+1}+C_{k}^{k+2}+...+C_{k}^{k+r}=C_{k+r-1}^{k+1}$

C $mC_{n}^{m}=nC_{n-1}^{m-1}$

D $C_{n}^{m-1}+C_{n}^{m}=C_{n+1}^m$