(1) Milk is not only rich in protein, protein is the basic substance of cells, but also rich in calcium, which can supplement teenagers and the elderly and avoid dysplasia or osteoporosis.

Therefore: protein; Calcium.

(2) Plastic fresh-keeping bags for food are synthetic organic polymer materials, which are not easy to decompose and easily cause "white pollution".

So, fill in: synthesis; Difficult.

(3) Cover the charcoal fire with sand. The fire extinguishing principle is to isolate combustible materials from oxygen.

Therefore, fill in: isolate oxygen.