Automobile engineers wanted to check the effect of a new automatic transmission on mileage. They selected 50 cars of the same make and model. Twenty-five of the cars used the old transmission, while the other 25 cars used the new model of transmission. A hypothesis test on the difference between the means was conducted. Which one of the following is a condition necessary to conduct this test?

a. The standard deviation of each group should be different.
b. The sampling distribution of the difference between the sample means is bimodal.
c. Since the data were random, no linear regression was possible.
d. Twenty-five cars are randomly selected with the new transmission and 25 with the old transmission.
e. The difference between the means was within one standard deviation.