Dr. Green is conducting a study to examine the effects of diet type (vegetarian, omnivorous) and exercise frequency (none, moderate, high) on weight loss. The results show that participants on a vegetarian diet lost more weight than those on an omnivorous diet, regardless of their exercise frequency. This finding demonstrates _____. a. an interaction between diet type and exercise frequency b. a confounding effect of diet type c. a main effect of exercise frequency d. a main effect of diet type e. no main effects