Select one or more correct answers.

Hakeem is testing the effects of sunlight on samples of soil. Which of these actions would introduce confounding variables into Hakeem’s experiment?

A. He tests a theory that’s already been established.
B. He tests multiple hypotheses at a time.
C. He uses identical specimens when completing multiple trials.
D. He adjusts the independent variable and records the effects.
E. He uses different types of soil in each of his samples.