Or a: salt b: acid c: element d: oxide e: base.

First of all, from "the reaction of C and B, C and D are both substitution reactions", we can know that A is a simple substance.

Simple substances can react with acids (iron and sulfuric acid) and oxides (hydrogen and copper oxide), so B and D are acids or simple substances.

Oxides can react with bases, such as sodium hydroxide and carbon dioxide.

Acids can react with salts such as hydrochloric acid and calcium carbonate.

So that we can get the answer.