A veterinarian wishes to compare the number of times race horses are reported to be lame during a 12‑month period, as compared with similar horses that are not raced. The veterinarian is particularly interested in whether the exercise schedule used in racing contributes to lameness, rather than the total amount of exercise. The veterinarian chooses nine race horses and selects, for each race horse, a jumping horse that is the same age, the same breed, and exercised a similar number of hours each week. The given table contains the observed number of times a horse was lame.

Group 1 2 3 4 5 6 7 8 9

Race 7 8 2 0 3 1 4 2 6

Jump 2 1 3 2 4 1 3 7 4

Please calculate the nine differences (race horse minus jumping horse) between the numbers of times the horses were lame in each group. A 90% confidence interval for the mean difference in the number of times a horse is lame is given by:

a. ( −3.405 , 4.738).
b. ( −2.459 , 3.792).
c. ( −1.59 , 2.923).
d. ( −1.329 , 2.663).