(2)A, the alloy is a metal material, not an organic synthetic material, so A is wrong;
B, cement belongs to silicate materials, not composite materials, so B is wrong;
C, the extensive use of synthetic materials has brought serious pollution to human beings, so it must be used reasonably, and production and use cannot be prohibited, so C is wrong;
D, the use of degradable plastics can effectively solve the problem of "white pollution", so D is correct.
So choose D.
(3)① Polyurethane is an organic synthetic material, which is flammable, so the answer is: organic; Flammability.
(2) Fireworks can ignite polyurethane and provide its combustion temperature, so B 。
③ Carbon monoxide is a toxic gas, which can combine with hemoglobin in blood; So the answer is: co.
(4) Fire extinguishing is the condition to block combustion. It can be seen from the narrative that above 380℃, magnesium hydroxide decomposes and absorbs the heat on the surface of combustible materials, releasing a lot of water, and the decomposed magnesium oxide covers the surface of combustible materials to prevent combustion, and the reaction of one substance generating two substances should be decomposition reaction;
So the answer is: the magnesium oxide decomposed by magnesium hydroxide covers the surface of combustible materials and isolates the air; Mg(OH)2？ △.？ MgO+H2O↑； Decomposition reaction.