When an odorant binds to an olfactory receptor, a cascade event triggers a cellular response, and the organism is able to detect smells. Use the diagram of this process to respond:

A diagram representing the initial cellular response when an odorant molecule binds to a receptor protein. The diagram shows the sequence of events from 1 - the odorant molecule binds to the receptor protein, 2 - the active adenylate cyclase occurs, 3 - the cAMP released opens the active Na+ and Ca2+ channel, 4 - the Ca 2+ ions enter the cell, 5 - the Ca 2+ opens the Cl- channel, 6 - the Ca2+ allows the Na+ to leave via the Na+/Ca2+ exchanger.
© 2012 FLVS

Using the information in the diagram, what is the primary reason the cascade event is occurring in the cell?

ATP is available to provide energy.
There is a greater concentration of calcium ion inside the cell.
There is only one type of receptor protein.
The receptor protein is specific for the odorant molecule.