Aaliyah has executed an experiment with 4 groups of pigs examining the effects of various lengths (time) of ultrasound exposure on wound healing. Each group consisted of 5 pigs each. After analyzing the experiment, Aaliyah found that MS between was slightly smaller than MS within. This indicates _______________.
a. a significant effect of ultrasound length (time) on wound healing
b. pigs were the incorrect animal species to use for this experiment
c. no significant effect of ultrasound length (time) on wound healing
d. a negative control group was needed