Which of the following oxo acid has two P-H bonds and one P-OH bond ?

• A

  ${\mathrm{H}}_3{\mathrm{PO}}_3$

• B

  ${\mathrm{H}}_3{\mathrm{PO}}_4$

• C

  ${\mathrm{H}}_3{\mathrm{PO}}_2$

• D

  ${\mathrm{H}}_3{\mathrm{PO}}_5$