Dr. Bonne DeMuth wanted to find out if a memory-enhancing drug would help decrease the time it takes students to learn how to trace a maze. She predicted that the students who received the memory-enhancing drug in the experiment would trace the maze more quickly than the students who did not receive the drug. She selected 20 students at random from the JJC student population and had her lab assistant randomly select and inject 10 of the students with the memory-enhancing drug and the other 10 students with a shot of an inactive solution (a placebo). Dr. Bonne DeMuth was not informed which group of students was the group receiving the drug and which group of students was the group receiving the placebo shots, and the students did not know which group they were in until after the experiment had been completed. After the students were injected, Dr. Bonne DeMuth tested each group of students on the maze, testing one student at a time and timing how long it took each student to learn how to correctly trace the maze. She gave each student 5 opportunities to trace the maze, and then calculated the average time it took each student, as well as the average time for each group of students overall. Besides the difference in what the shots contained (the memory-enhancing drug or the placebo solution), both groups of students were treated exactly the same during the experiment. What is the independent variable in this study?