Calcium oxide interacts with each of two substances: 1) H2O and MgO 2) HCl and KOH 3) H2O and CO2 4) CO2 and NaOH

Answer: 3.
Explanation of the solution to the problem: calcium oxide is a basic oxide, therefore it can react only with water or with compounds with acidic properties (acidic, amphoteric oxides; acids, in this case with carbon monoxide (IV)):
CaO + H2O = Ca (OH) 2;
CaO + CO2 = CaCO3.

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