Let P be the probability of happening any event and Q its failure, then the total chance of $r$ success in $n$ trials is

Question

$\begin {array} {1 1} (A)\;n\: C_rP^rQ^{n-r} & \quad (B)\;n\: C_rP^{r-1}Q^{r+1} \\ (C)\;C\: C_rP^{r+1}Q^{r-1} & \quad (D)\;n\: C_{n+r}P^rQ^{n-r} \end {array}$