An experiment is conducted to estimate the difference in work productivity for employees under two conditions: with and without music playing in the background. Twenty employees volunteered to be part of the study. Ten of the volunteers were randomly assigned to work for one week with music playing in the background. The other 10 volunteers worked for one week without music playing in the background. Productivity is measured by counting the number of units produced for the week. Here are dotplots, which display the data: 2 dotplots. The number lines are labeled number of units produced and go from 14 to 25. With music: 14, 1; 15, 1; 16, 1; 17, 1; 18, 3; 19, 1; 20, 1; 21, 1. Without music: 19, 1; 20, 1; 21, 1; 22, 4; 23, 1; 24, 1; 25, 1. What is the appropriate inference procedure to be used to estimate the difference in the mean number of units produced by employees who work with and without music playing in the background? t confidence interval for a mean z confidence interval for a proportion t confidence interval for a difference in means z confidence interval for a difference in proportions