A major department store chain is interested in estimating the mean amount its credit card customers spent on their first visit to the chain's new store in the mall. Ten credit card accounts were randomly sampled and analyzed with the following results: X = $75.50 and S = 20. Assuming the distribution of the amount spent on their first visit is normal, what is the shape of the sampling distribution of the sample mean that will be used to create the desired confidence interval for ?

A.

Approximately normal with a mean of $75.50

B.

A standard normal distribution

C.

A t distribution with 9 degrees of freedom

D.

A t distribution with 10 degrees of freedom