Arandomized, controlled experiment investigated this question using a random sample of 540 adult males as subjects. at the beginning of the study, researchers measured the blood pressure of all the subjects. each subject was given a month's supply of either drug a, drug b, or a placebo, then was assigned to be on either a low-fat diet or a low-salt diet for that month-long period. at the end of the month, the researchers measured the blood pressure of each subject again, and compared the two blood pressure readings. how many different groups are there altogether in this study?