Benzalkonium chloride ($A$) is a weak germicide used in topical antiseptics and mouthwash. $A$ can be prepared from amines $B$ or $C$ by S$_N$2 reaction with an alkyl chloride. (a) What alkyl chloride is needed to prepare $A$ from $B$? (b) What alkyl chloride is needed to prepare $A$ from $C$?