(1) The following five common foods are rich in protein ______. ① barbecued meat ② eggs ③ milk ④ doughnuts ⑤ cucumber...

(1) barbecue, eggs, milk is rich in protein, doughnuts contain starch and oil, cucumbers are rich in vitamins;

(2) according to the table can be seen barbecue, eggs, milk contains the elements Ca, Fe, Zn;

(3) ① calcium belongs to the macronutrient elements, iron belongs to the trace elements;

② iron supplementation is conducive to the maintenance of the blood in the body iron;

③ iodine is an important component of thyroxine;

④ calcium is mainly found in the human body's bones and teeth;

(4) ① melamine consists of three elements: carbon, nitrogen, and hydrogen;

② the mass ratio of the carbon and hydrogen elements in melamine is (12×3): (1×6) = 6:1;

③ the melamine's relative molecular mass is = 12 × 3 + 1 × 6 + 14 × 6 = 126;

④ the mass fraction of nitrogen contained in melamine is

14×6

126

×100%≈66.7%.

The answers are: (1) ①②③; (2) Ca, Fe, Zn; (3) ④; (4) ②④.