A Machine That Is Supposed to Fill Small Bottles to Contain

A machine that is supposed to fill small bottles to contain 20 milliliters yields the following data from a test of 4 bottles every hour. What would be the centerline for a control chart for $\bar { x }$ ?
$\quad$ $\quad$ $\quad$ $\quad$ $\quad$ $\text { Bottle Volume (mL) }$
$\begin{array}{c|cccc|c|c}\text { Sample }&&&&&\bar{x}&\text { Range }\\\hline1 & 19.9 & 20.1 & 20.2 & 20.3 & 20.125 & 0.4 \\2 & 20.4 & 20.0 & 20.3 & 20.3 & 20.250 & 0.4 \\3 & 20.0 & 20.7 & 20.4 & 20.3 & 20.350 & 0.7 \\4 & 20.4 & 20.1 & 20.1 & 19.9 & 20.125 & 0.5 \\5 & 19.9 & 19.8 & 19.6 & 19.5 & 19.700 & 0.4 \\6 & 19.4 & 19.4 & 19.6 & 19.7 & 19.525 & 0.3 \\7 & 19.8 & 19.4 & 19.6 & 19.7 & 19.625 & 0.4 \\8 & 19.9 & 19.8 & 20.0 & 20.0 & 19.925 & 0.2 \\9 & 20.2 & 20.3 & 20.1 & 20.3 & 20.225 & 0.2 \\10 & 20.0 & 20.3 & 20.0 & 20.2 & 20.125 & 0.3 \\11 & 20.3 & 20.5 & 20.1 & 20.2 & 20.275 & 0.4 \\12 & 20.1 & 19.9 & 19.8 & 19.7 & 19.875 & 0.4 \\13 & 19.5 & 19.8 & 19.7 & 19.6 & 19.650 & 0.3 \\14 & 19.4 & 19.8 & 19.8 & 19.4 & 19.400 & 0.4 \\15 & 19.5 & 19.6 & 19.6 & 19.9 & 19.650 & 0.4\end{array}$

A) $0.380$ milliliters
B) $19.935$ milliliters
C) $20.000$ milliliters
D) $20.212$ milliliters