Answer:
The covalent bond between the middle phosphate and the last phosphate.
Step-by-step explanation:
When the student builds the model of an ATP molecule form some scraps.She also used a block of wood for the bulk of the molecule labelled as ADP.So when the block is connected to the spring and then compressed between the block and a paperweight here the coil spring represents the covalent bond between the middle and the last phosphate.