Two Phosphoric Acid Molecules Can Combine to Make Pyrophosphoric Acid

Two phosphoric acid molecules can combine to make pyrophosphoric acid and another molecule as shown here. The pyrophosphoric acid molecule is a phosphoester and is a structural element in the ADP/ATP metabolic energy system. What molecule is represented by the question mark in the structural reaction equation below?

A) O₂
B) OH^-
C) H₂O
D) PO
E) H₂