Carlos, the president of a fraternity on campus, randomly sampled 500 students, asked them if they ...

Carlos, the president of a fraternity on campus, randomly sampled 500 students, asked them if they belonged to a fraternity or sorority, and asked them about their current GPA.
 
  He discovered that the GPAs of those people in fraternities and sororities were higher than those of people who were not involved in the Greek system. Gleefully, he presented his findings to the dean, saying that being involved in a fraternity or sorority leads to higher grades. What rule of research methods is Carlos breaking?
 
   a. Correlation does not prove causation.
   b. College students are not representative of the whole sample of people in the world.
   c. His sample size of 500 is too small to make such a generalization.
   d. Correlational data do not provide any practical information on a topic.