Some Research Questions Cannot Be Answered Using Human Experiments Because

Multiple Choice

Some research questions cannot be answered using human experiments because

A) some studies would be impractical or unethical.
B) randomization is often hard to achieve.
C) double-blind studies are usually dangerous to participants.
D) researchers know that their results usually will not be published.
E) they are not a very effective way to learn about cause-and-effect relationships.