Classify each of the following reactions as a combination reaction, decomposition reaction, displacement reaction, or combustion reaction.

a. When they are heated, ammonium dichromate crystals, (NH₄)₂Cr₂O₇, decompose to give nitrogen, water vapor, and solid chromium(III) oxide, Cr₂O₃.

b. When aqueous ammonium nitrite, NH₄NO₂, is heated, it gives nitrogen and water vapor.

c. When gaseous ammonia, NH3, reacts with hydrogen chloride gas, HCl, fine crystals of ammonium chloride, NH₄Cl, are formed.

d. Aluminum added to an aqueous solution of sulfuric acid, H₂SO₄, forms a solution of aluminum sulfate, Al₂(SO₄)₃. Hydrogen gas is released.