What type of compounds are formed when reactive metals like sodium and potassium react with dihydrogen?

$A, B$ and $C$ are three elements which undergo chemical reactions according to the following equations:
$A_{2}O_{3} + 2B \rightarrow B_{2}O_{3} + 2A$
$3CSO_{4} + 2B \rightarrow B_{2}(SO_{4})_{3} + 3C$
$3CO + 2A \rightarrow A_{2}O_{3} + 3C$
$(a)$ Which element is the most reactive?
$(b)$ Which element is the least reactive?
$(c)$ What is the type of reactions listed above?

Which of the following gases is insoluble in water?

Why do ionic compounds not conduct electricity in the solid state?

Which among the following alloys contains mercury as one of its constituents?

What happens when copper oxide is heated in the presence of dihydrogen gas?