In order to determine if engine oil EEEA B C will improve gas mileage, a research study was conducted. From a wide range of vehicle makes, models, and model years, 400 vehicles were randomly selected. Half of the vehicles were chosen at random to receive oil , and the other half received a generic brand of oil. The resulting data showed that vehicles that received engine oil had substantial improvements in gas mileage as compared to those that received the generic brand of engine oil. Which of the following can best be concluded based on the design and results of the research?
a. Engine oil E improves gas mileage better than any other available engine oil.
b. Engine oil E will cause a significant improvement in gas mileage.
c. Engine oil E will improve the gas mileage of any vehicle that uses it.
d. Engine oil E is likely to improve the gas mileage of cars that use it.

The research was conducted with the random vehicles who were introduced with Engine oil E. When compared with engine oil E and the generic oil the vehicle that uses engine oil E performed better. The gas mileage of such vehicles improved significantly as compared to other vehicles who received generic oil.