If $N$ denotes the number of ways of selecting $r$ objects of out of $n$ distinct objects $\left(r\ge n\right)$ with unlimited repetition but with each object included at least once in selection, then $N$ is equal is a. ${.}^{r-1}{C}_{r-n}$ b. ${.}^{r-1}{C}_n$ c. ${.}^{r-1}{C}_{n-1}$ d. none of these

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