An advertiser was about to launch an advertisement campaign for a new product. He conducted a random survey to find the preference for the new product among the respondents. Based on his previous experience, he found the standard deviation for the population as 4. Because the population mean was not known to him, he randomly selected 25 responses and found that the sample mean preference for the new product was 10. Which of the following parameters did he use to calculate a confidence interval at the 95% confidence level for this population mean? ​