(2.5 bonus points) The head librarian at the Library of Congress has asked her assistant for an interval estimate of the mean number of books checked each day. The assistant provides the following interval estimate: from 740 to 920 books per day. If the head librarian knows that the population standard deviation is 150 books checked out per day, and she asked her assistant for 95% confidence interval, approximately how large a sample did her assistant use to determine the interval estimate? (a) 125 (b) 13 (c) 11 (d) 8 (2.5 bonus points) The researchers are not satisfied with their confidence interval and want to do another study to find a shorter confidence interval. What should they change to ensure they find a shorter confidence interval? (a) They should increase their confidence level and increase their sample size. (b) They should increase their confidence level but decrease their sample size. (c) They should decrease their confidence level but increase their sample size. (d) They should decrease their confidence level and decrease their sample size.