If $\bar{x}_{1}, \bar{x}_{2}, \bar{x}_{3}, \ldots, \bar{x}_{n}$ are the means of $n$ groups with $n_{1}, n_{2}, \ldots, n_{n}$ number of observations respectively,then the combined mean $\bar{x}$ of all the groups taken together is given by:

• A
  $\sum_{i=1}^{n} n_{i} \bar{x}_{i}$
• B
  $\frac{\sum_{i=1}^{n} n_{i} \bar{x}_{i}}{n^{2}}$
• C
  $\frac{\sum_{i=1}^{n} n_{i} \bar{x}_{i}}{2 n}$
• D
  $\frac{\sum_{i=1}^{n} n_{i} \bar{x}_{i}}{\sum_{i=1}^{n} n_{i}}$