Which compound reacts with calcium oxide, CaO?

A.     ${{\text{K}}_{\text{2}}}{\text{O}}$

B.     ${\text{N}}{{\text{a}}_{\text{2}}}{\text{O}}$

C.     ${\text{S}}{{\text{O}}_{\text{2}}}$

D.     MgO

C

This question assumed an understanding of Periodicity (3.3.2) and Acid-Base (8.2.1). The majority of candidates (69%) had no difficulty in determining that calcium oxide is a basic oxide and choosing the only acidic oxide, sulfur dioxide, as the answer.