The average of n numbers $x_{1}, x_{2}, ...., x_{n}$ is $\overline{x}$. Then the value of $\sum_{i=1}^{n}(x_{i} \overline{x})$ is equal to
[A] $0$
[B] $\overline{x}$
[C] $n\overline{x}$
[D] $n$