To test the effect of a new exercise routine on muscle growth, a researcher sets up the following  experiment: 60 volunteers subjects (40 men and 20 women) of average build are selected to participate in the  study. Their body mass and muscle sizes (biceps, thighs, etc.) are measured. The researcher gives all the men a DVD with the new exercise routine, and the women a DVD with a standard routine, but only the researcher knows this. The results of the experiment are likely to be invalid mostly because:
• the subjects were volunteers.
• the gender of the volunteer is a confounding variable in this experiment.
• the subjects did not know if they were getting the real treatment.
• none of the above.

It would improve the experiment if the researchers:
• gave the new workout to the women.
• gave everyone the same work out.
• did not use so many women.
• used the same number of men and women and gender-non-conforming subjects