$3$ moles of ethanol react with $1$ mole of phosphorus tribromide to form $3$ moles of bromoethane and $1$ mole of $X$. Which of the following is $X$?

• A
  $H_3PO_4$
• B
  $H_3PO_2$
• C
  $HPO_3$
• D
  $H_3PO_3$