C. Three

A third major advantage of a clinical trial is that conditions are controlled as carefully as possible, minimizing potential biases. Both sampling and measurement bias are greatly reduced, if not eliminated. In contrast, survey studies are generally subject to significant sampling and measurement bias.

LAST：Big promotion !! Apoloe vaporizer pen cloutank Ego-w e cigarette easy usable technology |NEXT：The newest vv vw mod vamo v5 kit ecig bluetooth Etunes