Which of the following compounds are isomers of propionic acid?

• A
  $HCOOC_2H_5$ and $CH_3COOCH_3$
• B
  $HCOOC_2H_5$ and $C_3H_7COOCH_3$
• C
  $CH_3COOCH_3$ and $C_3H_7OH$
• D
  $C_3H_7OH$ and $CH_3COCH_3$