A first aid cream contains amounts of the active ingredient in mg that vary ...

  

A first aid cream contains amounts of the active ingredient in mg that vary normally with unknown mean µ and standard deviation from tube to tube, you select a simple random sample of 9 tubes and assess the quantity of active ingredient they contain. The 9 amounts in mg are,

23   24   23   29   28   26   27   28   26

Based on these data, a 99% confidence interval for the population mean µhas margin of error

a.   2.50

b.   1.92

c.   2.42

d.   0.75