Identify $A, B, C,$ and $D$ in the following reaction sequence:
$CH_3-CH_2-CH(OH)-CH_3$ $\xrightarrow{PBr_3} (A)$ $\xrightarrow{Mg, \text{ether}} (B)$ $\xrightarrow{CH_3CHO} (C)$ $\xrightarrow{H_3O^+} (D)$

(N/A) The reaction sequence proceeds as follows:
$1$. $CH_3-CH_2-CH(OH)-CH_3 + PBr_3 \rightarrow CH_3-CH_2-CH(Br)-CH_3 (A)$ (Substitution of $-OH$ with $-Br$)
$2$. $CH_3-CH_2-CH(Br)-CH_3 + Mg \xrightarrow{\text{ether}} CH_3-CH_2-CH(MgBr)-CH_3 (B)$ (Formation of Grignard reagent)
$3$. $CH_3-CH_2-CH(MgBr)-CH_3 + CH_3CHO \rightarrow CH_3-CH_2-CH(CH_3)-CH(OMgBr)-CH_3 (C)$ (Nucleophilic addition to acetaldehyde)
$4$. $CH_3-CH_2-CH(CH_3)-CH(OMgBr)-CH_3 + H_3O^+ \rightarrow CH_3-CH_2-CH(CH_3)-CH(OH)-CH_3 (D)$ (Hydrolysis to form secondary alcohol)