Javier wants to study the effects on achievement of taking a course in chemistry in the afternoon, rather than in the morning. A teacher has chemistry classes with the same number of students at 8:30 A.M. and 1:00 P.M., and volunteers to participate with her classes. A major problem in this study would be
(A)poor replication (B)lack of a hypothesis (C)confounding variables (D)difficulty in obtaining informed consent
C