Which of the following issues complicate the use of unobtrusive measures?

A) There is no way to establish clearly the reliability of such measures.
B) There is no way to establish clearly the validity of such measures.
C) A person's behavior will be distorted by the use of unobtrusive measures, making generalizability to other settings impossible.
D) There is a potential ethical problem in measuring a person's behavior without their knowledge or permission.

Answer: D