A Manufacturer of Silk Inspects Each Bolt of Material at the End

Multiple Choice

A manufacturer of silk inspects each bolt of material at the end of the manufacturing process. When the process is in control, it averages 11 defects per bolt. If a 2- sigma control limit is used, what is the upper control limit?

A) less than or equal to 15
B) more than 15 but less than or equal to 17
C) more than 17 but less than or equal to 19
D) more than 19