Which of the Following Situations Is Least Representative of Experimental

Which of the following situations is least representative of experimental research?

A) Five samples are drawn from a population, and each sample trains with a different method. Then differences among samples are determined.
B) Two samples are drawn from a population. One sample receives a treatment, and the other sample receives no treatment. Differences between samples are determined.
C) From a population, a sample is drawn. Participants are tested before and after receiving a treatment. The differences in their two scores are determined.
D) From a population, a sample is drawn. One half of the sample receive treatment A and then treatment B, while the other half receive the treatments in reverse order. The difference in the two treatments is determined.
E) Three samples are drawn from a population. Participants are tested, and differences among samples is determined.