What is the role of the metal or reagents written on the arrows in the given chemical reactions?
$(a)$
$(b)$ $CH_{3}COOH + CH_{3}CH_{2}OH \xrightarrow{Conc. H_{2}SO_{4}} CH_{3}COOC_{2}H_{5} + H_{2}O$
$(c)$ $CH_{3}CH_{2}OH \xrightarrow[Heat]{Alkaline KMnO_{4}} CH_{3}COOH$

(N/A) $Ni$ (Nickel) acts as a catalyst in the hydrogenation reaction.
$(b)$ Concentrated $H_{2}SO_{4}$ (Sulfuric acid) acts as a dehydrating agent and a catalyst in the esterification reaction.
$(c)$ Alkaline $KMnO_{4}$ (Potassium permanganate) acts as an oxidizing agent.