A major department store chain is interested in estimating the average amount its credit card customers spent on their first visit to the chain’s new store in the mall. Fifteen credit card accounts were randomly sampled and analyzed with the sample mean 50.5 dollars and sample variance 400. Construct a 95% confidence interval for the average amount its credit card customers spent on their first visit to the chain’s new store in the mall, assuming that the amount spent follows a normal distribution.
选项：

A:$50.50 ± $11.08;
B:$50.50 ± $9.09;
C:$50.50 ± $10.12;
D:$50.50 ± $11.00