Amajor retail clothing store is interested in estimating the difference in mean monthly purchases by customers who use the store's in-house credit card versus using a visa, mastercard, or one of the other major credit cards. to do this, it has randomly selected a sample of customers who have made one or more purchases with each of the types of credit cards. the following represents the results of the sampling:

in-house credit card national credit card
sample size: 86 113
mean monthly purchases: $45.67 $39.87
standard deviation: $10.90 $12.47
suppose that the managers wished to test whether there is a statistical difference in the mean monthly purchases by customers using the two types of credit cards, using a significance level of .05, what is the value of the test statistic assuming the standard deviations are known?

a. z = 11.91
b. z = 2.86
c. t = 3.49
d. z = 3.49