Which among the following is a pair of functional isomers?

• A
  Butan-$2$-ol and $2$-Methylpropan-$1$-ol
• B
  Butan-$1$-ol and $1$-Methoxypropane
• C
  $1$-Methoxypropane and Ethoxyethane
• D
  $2$-Methoxypropane and $1$-Methoxypropane