The Presence of Caffeine,because of Its Bitter Taste,has Long Been

The presence of caffeine,because of its bitter taste,has long been considered a plant defense strategy,yet some coffee and citrus plants produce nectar with caffeine concentrations below the taste threshold of their pollinators,yet high enough to modify the pollinator's learning and memory.What hypothesis best explains this phenomenon?

A) The plants and their pollinators are coevolving; the pollinators will frequent and pollinate the caffeine-producing plants at higher rates than pollinators whose memory is unaffected by caffeine.
B) These plants once produced wide ranges of caffeine, but only those with moderate concentrations were pollinated; the pollinators are the directional pressure for these plants.
C) The pollinators once responded to the toxicity of caffeine with a variety of tolerances, but the benefits of caffeinated nectar have become the directional pressure for these insects.
D) Insect pollinators have poor vision, smell, and directional sense; without memory enhancements, they are almost incapable of returning to the same location.