For a repeated-measures study with n 5 12 scores in each treatment, a researcher constructs an 80% confidence interval to describe the mean difference between treatments. What value is at the center of the interval and what t values are used to construct the interval?

a. The sample mean difference is at the center and t 5 61.363.

b. The sample mean difference is at the center and t 5 61.316.

c. Zero is at the center and t 5 61.363.

d. Zero is at the center and t 5 61.316.