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 subjects each. Group 1 kicked a football filled with helium to the recommended pressure. Group 2 kicked a football filled with air to the recommended pressure. The mean yardage for group 1 was= 30 yards, with a standard deviations1= 8 yards. The mean yardage for group 2 was= 26 yards, with a standard deviations2= 6 yards. Assume the two groups of kicks are independent. Let1and2represent the mean yardage we would observe for the entire population represented by the volunteers if all members of this population kicked, respectively, a helium- and an air-filled football. Assume that two-samplet-procedures are safe to use.

Reference: Ref 17-2

A 99% confidence interval for1-2is (use the conservative value for the degrees of freedom)

Select one:

a.-8.7 to 16.7 yards.

b.-0.7 to 8.7 yards.

c.-2.2 to 10.2 yards.

d.-7.2 to 15.2 yards.