Why Are Randomized Field Experiments Considered the "Gold Standard" of Evaluation

Why are randomized field experiments considered the "gold standard" of evaluation research?

A) They usually take place in a laboratory,where conditions can be closely controlled.
B) They tend to have a higher rate of participation than other research studies.
C) They are not biased by the differences between treatment and control groups.
D) They are relatively easy to administer compared to other sociological research methods.