A researcher wishes to estimate, with 90 % confidence, the population proportion of adults who think the president of their country can control the price of gasoline. Her estimate must be accurate within3 % of the true proportion. a) No preliminary estimate is available. Find the minimum sample size needed. b) Find the minimum sample size needed, using a prior study that found that 28 % of the respondents said they think their president can control the price of gasoline. c) Compare the results from parts (a) and (b).