Paul Solomon and His Colleagues Investigated the Effects of the Herb

Paul Solomon and his colleagues investigated the effects of the herb ginkgo biloba on memory, concentration, and other mental tasks. All participants in the study took a battery of cognitive tests at the beginning and end of the six-week study. The results showed that the test scores increased about the same for the participants in the experimental group as for the participants in the placebo control group. How is this best explained?

A) The researchers did not use a double-blind technique in the study, and therefore the results may have been influenced by demand characteristics.
B) Because the researchers failed to use random assignment the ginkgo biloba group may have had more "health conscious" participants than the placebo group, which may have biased the results.
C) From simply taking the cognitive tests twice the participants in both groups experienced a practice effect that improved their test scores.
D) Because the test scores had improved by the end of the experiment it can be concluded that ginkgo biloba significantly improves memory, alertness, concentration, and mental focus.