A Quality Analyst Wants to Construct a Sample Mean Chart

A quality analyst wants to construct a sample mean chart for controlling a packaging process. He knows from past experience that whenever this process is in control, package weight is normally distributed with a mean of 20 ounces and a standard deviation of two ounces. Each day last week, he randomly selected four packages and weighed each: $$\begin{array} { l l c c c } \text { Day } && { \text { Weight (ounces) } } \\\hline \text { Monday } & 23 & 22 & 23 & 24 \\\text { Tuesday } & 23 & 21 & 19 & 21 \\\text { Wednesday } & 20 & 19 & 20 & 21 \\\text { Thursday } & 18 & 19 & 20 & 19 \\\text { Friday } & 18 & 20 & 22 & 20\end{array}$$
What is the standard deviation of the sampling distribution of sample means for whenever this process is in control?

A) 0.1 ounces
B) 0.4 ounces
C) 0.5 ounces
D) 1 ounce
E) 2 ounces