Giovanni Was Watching the Night Sky on a Clear Evening

Giovanni was watching the night sky on a clear evening in November. He noticed that sometimes when he looked directly overhead he could detect a very faint star. A few minutes later it seemed that the star had disappeared, and then it "appeared" again. How would you describe the light from the star, in this case?

A) It produces action potentials that were too weak to reach the terminal buttons.
B) It produces inhibitory potentials rather than excitatory potentials.
C) It falls just below Giovanni's level for perceptual invariance.
D) It is just at Giovanni's absolute threshold for light.