(1) The digestion of starch begins in the mouth, but the stomach cannot digest starch, and starch is completely digested into glucose in the small intestine. Neither the mouth nor the stomach can digest fat. Fat is digested in the small intestine. There are many digestive juices in the small intestine, such as bile, pancreatic juice, intestinal juice, etc., which contain enzymes for digesting sugar, protein and fat. Although bile does not contain digestive enzymes, it emulsifies fat, turns fat into tiny particles, increases the contact area with digestive enzymes, and thus promotes fat digestion. Fat is completely decomposed into glycerol and fatty acids in the small intestine; Therefore, the digestion of starch and fat corresponds to the y and x curves in the figure respectively.

(2) The digestive tract shown in the figure basically has no digestive function, that is, B esophagus, and the D segment of the digestive tract represents the small intestine, which is about 5-6 m long and contains various digestive juices such as intestinal juice, pancreatic juice and bile; There are annular folds on the inner wall of the small intestine, and there are small intestinal villi on the folds, which increases the area of digestion and absorption; There are capillaries and lymphatic capillaries in the villi of small intestine, which are composed of a layer of epithelial cells, which is beneficial to the absorption of nutrients into capillaries and lymphatic capillaries in the inner wall of small intestine. Therefore, the small intestine is the main place to digest food and absorb nutrition.

(3) As can be seen from the figure, the initial digestion sites of starch, protein and fat are oral cavity, stomach and small intestine respectively.

So, the answer is: (1) y; ? x；

(2)B？ Esophagus; D small intestine;

(3) small intestine; stomach