A sports writer wished to see if a football filled with helium travels farther, on average, than a football filled with air. To test this, the writer used 18 adult male volunteers. These volunteers were randomly divided into two groups of nine men each. Group 1 kicked a football that was filled with helium to the recommended pressure. Group 2 kicked a football that was filled with air to the recommended pressure. The mean yardage for Group 1 was x¯1=300 yards with a standard deviation of s1=8 yards. The mean yardage for Group 2 was x¯2=296 yards with a standard deviation of s2=6 yards. Assume the two groups of kicks are independent. Let µ1 and µ2 represent the mean yardage we would observe for the entire population represented by the volunteers if all members of this population kicked, respectively, a helium‑filled football and an air‑filled football. Assuming two‑sample t procedures are safe to use and using Option 1 for the degrees of freedom, a 90% confidence interval for µ1−µ2 is:.
a. 4 ± 5.5 yards.
b. 4 ± 6.2 yards.
c. 4 ± 7.7 yards.
d. 4 ± 11.2 yards.