While Conducting an Experiment on Alcohol Tolerance, Dr

While conducting an experiment on alcohol tolerance, Dr. Guff solicits his participants at the door to Skam's (a local drinking and social establishment) . When he publishes his findings (that people are extremely tolerant to the effects of alcohol) , Dr. Guff's research is criticized because

A) there are too many dependent variables.
B) he used a double-blind design.
C) the sample is biased.
D) he used a double-blind design and the samples are biased.