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egin{tabular}{cc} $\newline$Time (minutes) & Number of runners \\ $\newline$\hline $18$ & $1$ \\ $\newline$$19$ & $2$ \\ $\newline$$20$ & $3$ \\ $\newline$$21$ & $6$ \\ $\newline$$22$ & $r$ \\ $\newline$$23$ & $1$ \\ $\newline$$24$ & $2$ $\newline$\end{tabular}$\newline$The time in minutes, rounded to the nearest minute, that runners took to finish a $3$-mile race is shown in the table. If the mean finish time was $21$ minutes, what is the value of $r$ in this table?$\newline$(A) $1$$\newline$(B) $2$$\newline$(C) $3$$\newline$(D) $6$