A psychologist would like to know how many casual friends are in the average person's social network. She interviews a random sample of people and determines for each the number of friends or social acquaintances they see or talk to at least once a year. The data are as follows: 5,11,15,9,9,7,13,23,8,12,7,10,11,21,20,13.

A. Find the 90% confidence interval for the mean number of friends for the entire population?

B. Find the 95% CI?

C. If a previous researcher had predicted a population mean of 10 casual friends per person, could that prediction be rejected as an hypothesis at the 0.5 level, two-tailed?