The dollar value of orders placed through a particular store catalog is found to be normally distributed with a standard deviation of $21. A sample...

The dollar value of orders placed through a particular store catalog is found to be normally 

distributed with a standard deviation of $21.58. A sample of 30 orders averaged $81.25. What is 

the 90% confidence interval around this sample mean? 

A) $81.25 ± $10.79 B) $81.25 ± $21.58 C) $81.25 ± $12.15 D) $81.25 ± $6.48