(1)Z is a gas that can produce "greenhouse effect" on the earth, so Z is carbon dioxide, A is alkali, which can react with magnesium-containing compound X to generate white precipitate I and sodium chloride, then A is sodium hydroxide, white precipitate I is magnesium hydroxide, X is magnesium chloride, and white precipitate II can react with B to generate magnesium carbonate and carbon dioxide, then white precipitate II may be magnesium carbonate, B may be hydrochloric acid, and C may be sodium carbonate.

(2) Magnesium chloride can react with sodium carbonate to produce magnesium carbonate precipitate and sodium chloride, so the answer to this question is: MGC12+Na2CO3 ═ 2NaCl+MgCO3 ↓