Mack Trucks Decides to Conduct an Experiment to Determine If $\mathrm{X} \quad \mathrm{O}_{1}$

Mack Trucks decides to conduct an experiment to determine if a new oil additive actually results in increased fuel efficiency. They find a lab that contains 50 engines that are exactly alike. Mack randomly divides the 50 engines into two groups an experimental group (which has the new oil additive) and the control group (which does not have the new oil additive) . They pour the exact same amount of fuel into all 50 engines and time how long each engine runs. Afterward, they average the length of time run by the 25 engines in the control group versus the 25 engines in the experimental group. Which of the following represents the experimental design in this study?

A) $\mathrm{X} \quad \mathrm{O}_{1}$
B) $\text { (R) } \mathrm{O}_{1} \mathrm{X} \quad \mathrm{O}_{2}$
$\begin{array}{ll}\mathrm{O}_{3} & \mathrm{O}_{4}\end{array}$
C) $\mathrm{O}_{1} \mathrm{X} \quad \mathrm{O}_{2}$
D) $\text { (R) } \mathrm{O}_{1} \mathrm{X} \quad \mathrm{O}_{2}$
$\text { (R) } \mathrm{O}_{3} \quad \mathrm{D} \quad \mathrm{O}_{4}$
E) $\text { (R) } \mathrm{O}_{1} \quad \mathrm{O}_{2}$
$\begin{array}{lll}\mathrm{O}_{3} & \mathrm{D} & \mathrm{O}_{4}\end{array}$