Use the Following Information for the Question Below

Use the following information for the question below:
You conduct a clinical trial to test whether a new drug relieves the symptoms of arthritis better than a placebo.You have four groups of participants,all of whom have moderately painful arthritis (rated 7 on a scale of 1 to 10) .Each group receives a daily pill as follows: group 1-placebo;group 2-15 mg;group 3-25 mg;group 4-50 mg.At the end of 2 weeks,participants in each group are asked to rate their pain on a scale of 1 to 10.
What is the independent variable in this experiment?

A) the level of pain experienced
B) the different drug treatment groups
C) the degree to which symptoms changed
D) the drug itself
E) the amount of pain experienced at the end of the experiment by the different drug treatment groups