Tim sets up an experiment with 3 groups of runners. group 1 smokes 1 pack per day. group 2 smokes 2 packs per day. group 3 smokes 3 packs per day. tim does a study to determine whether or not cigarette smoking decreases speed in track runners. after 1 month, tim measures the average 40 yard dash time for each group and finds that group 3 was the slowest, 2 was in the middle, and 1 was the fastest. he concludes that cigarette smoking decreases speed, but this is not necessarily correct. what was wrong with his experiment? a) tim measured more than one independent variable. b) tim did not have a dependent variable. c) tim did not use a control group. d) tim did not have a hypothesis.