You Carry Out a Clinical Trial to Test Whether a New

You carry out 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 mildly painful arthritis (rated 6 on a scale of 1 to 10) .Each group receives a daily pill as follows: group 1 (control) ,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 amount of pain experienced at the start of the experiment
B) the amount of pain experienced at the end of the experiment
C) the degree to which pain symptoms changed between the start and the end of the experiment
D) the drug
E) All of the answers could be the independent variable.