In the following reaction sequence,$CH_3CH_2OH$ $\xrightarrow{P + I_2} A$ $\xrightarrow[\text{dry ether}]{Mg} B$ $\xrightarrow{HCHO} C$ $\xrightarrow{H_2O} D$,the compound $D$ is:

• A
  Butanal
• B
  $n$-Butyl alcohol
• C
  $n$-Propyl alcohol
• D
  Propanal