A research study is planned to determine the effect of nutritional supplementation and type of support surface on the rate of wound healing in sacral myocutaneous flap failures. Which are independent variables in this study?

Crops grew well in mesopotamia because a. the land was in mountainous areas. b. farmland was fertile and water was nearby. c. the area was near the ocean water supply. d. the area received adequate rainfall. select the best answer from the choices provided a b c d