Compound A reacts with ${\mathrm{PCl}}_5$ to give B which on treatment with $\mathrm{KCN}$ followed by hydrolysis gave propionic acid. What are A & B respectively ?

After reacting a compound "A" with phosphorus pentachloride $\left(PC{l}_5\right)$, a compound "B" an alkyl halide is formed, which then reacts with $\mathrm{KCN}$ to form cyanide, which then hydrolyzes to form acid. The reaction:

A naturally occurring carboxylic acid called propionic acid has an unpleasant odor and is colorless and corrosive in its pure form.