Dr. Denton designs an experiment to test the hypothesis that children who are taught alternative ...

Dr. Denton designs an experiment to test the hypothesis that children who are taught alternative ways of responding to failure will show increased persistence in problem solving. Dr. Denton assigns half of a random sample of third-graders to a tutor who emphasizes learning from mistakes rather than improving grades. The other half of the sample receive tutoring without this emphasis on learning goals. After two months of tutoring, both groups were given a test of persistence. A possible random variable in this experiment is the ____.
 
  a. amount of time before the persistence test
  b. persistence test score
  c. level of intelligence of the children
  d. time of day the experiment was conducted