(1) According to the balanced diet pagoda, it can be seen that the first-class food in the balanced diet pagoda is rice, bread, potatoes and other foods, and the main nutritional component is starch. Starch is digested from the mouth, decomposed into maltose by salivary amylase, and completely decomposed into glucose in the small intestine.

(2) ① The liver in Figure B is the largest digestive gland in human body, and the adult liver weighs about 1.5kg, most of which is located in the upper right part of abdominal cavity. The liver secretes bile and has the function of emulsifying fat.

(3) protein is the basic substance that constitutes human cells; The growth and development of human body and the renewal of tissues are inseparable from protein; Teenagers are in a critical period of growth and development, so they should eat more third-and fourth-grade food in the "Balanced Diet Pagoda", which is rich in protein and protein.

(4) The small intestine is the place to digest food and absorb nutrition. This is because the small intestine 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.

(5) Xiaodong often eats rice, braised crucian carp, scrambled eggs with onions or tofu for lunch. Judging from Xiaodong's diet, his food contains starch, protein, vitamins and other substances. Vitamin C can prevent scurvy, so take it in moderation.

So the answer is: (1) starch (or sugar); Oral cavity. (2) bile. (3) protein; ②. ④ Small intestine. (5) C。