Write the structure of pyrophosphoric acid.

(N/A) The chemical formula of pyrophosphoric acid is $H_4P_2O_7$.
Its structure consists of two $PO_4$ tetrahedral units linked by an oxygen atom ($P-O-P$ linkage).
Each phosphorus atom is bonded to one terminal oxygen atom via a double bond $(P=O)$ and two hydroxyl groups $(-OH)$.
The structure is as follows:
$HO-P(=O)(OH)-O-P(=O)(OH)-OH$