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? 
Definitions:
Bayley-III
A standardized assessment tool used to measure the developmental progress of infants and toddlers across cognitive, language, and physical domains.
Fine Movement
The coordination of small muscle movements, often involving skills such as writing, typing, or fastening buttons, which require precision and control.
Bayley-III
The third edition of the Bayley Scales of Infant and Toddler Development, a standard series of measurements used for assessing the development of infants and toddlers.
Upright Position
The posture of an organism being in a vertical stance, often associated with the evolution of humans.
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