You have noticed that your next-door neighbor, Mary, always works in the garden, and her husband, Joe, always walks the dog. You conclude that if Joe and Mary are efficient, then it must be the case that:

A、Mary's opportunity cost of walking the dog is lower than Joe's.;
B、Joe experiences increasing opportunity costs when he gardens, but not when he walks the dog.;
C、Joe has a comparative advantage in walking the dog.;
D、Mary has an absolute advantage in gardening.