When Calculating the Confidence Interval for an Average,the Standard Deviation

When calculating the confidence interval for an average,the standard deviation is used:

A) because it indicates lack of variability
B) because it is the appropriate measure of variability for a metric variable
C) because it is the appropriate measure of variability for a categorical variable
D) because it is smaller and less expensive to calculate
E) none of the above; standard deviation is not used to calculate confidence for an average