Classical Studies Are Typically Done in Laboratories

Classical studies are typically done in laboratories. When we do evaluation research in real-life settings, what can happen?

A) Your results are invalid if all forms of bias were not able to be ruled out.
B) Your study could become nonreplicable, and thus meaningless.
C) You may not be able to conclude that your results were caused by the treatment and not by something else.
D) Your research may be deemed unethical if it affects anyone other than your subjects.