Give the structures of $A$,$B$,and $C$ in the following reaction:
$CH_3-CH_2-CH(OH)-CH_3$ $\xrightarrow{Cu, 520 \ K} (A)$ $\xrightarrow{C_2H_5MgBr} (B)$ $\xrightarrow{H_3O^+} (C)$

(A) $1$. The starting material is butan$-2-$ol,a secondary alcohol. Dehydrogenation of secondary alcohols with $Cu$ at $520 \ K$ yields a ketone. Thus,$(A)$ is butan$-2-$one: $CH_3-CH_2-CO-CH_3$.
$2$. The reaction of butan$-2-$one with ethylmagnesium bromide $(C_2H_5MgBr)$ is a Grignard reaction. The nucleophilic ethyl group attacks the carbonyl carbon,forming an intermediate alkoxide: $(B)$ is $CH_3-CH_2-C(OMgBr)(CH_3)-C_2H_5$.
$3$. Acidic hydrolysis of the intermediate $(B)$ with $H_3O^+$ yields the corresponding tertiary alcohol: $(C)$ is $CH_3-CH_2-C(OH)(CH_3)-C_2H_5$ ($3$-methylpentan$-3-$ol).