Which one of the following substances is used in the laboratory for a fast drying of neutral gases?

Which of the following oxides cannot be reduced by hydrogen?

Which is an example of a molecular hydride?

Which is the lightest gas?

Calcium imide on hydrolysis gives gas $(B)$ which on oxidation by bleaching powder gives gas $(C)$. Gas $(C)$ on reaction with magnesium gives compound $(D)$ which on hydrolysis again gives gas $(B)$. Identify $(B)$,$(C)$ and $(D)$.

Fire extinguishers contain $H_2SO_4$ and which one of the following?