In Engelmann's Classic Experiment, Why Were the Oxygen-Requiring Bacteria Clustered

In Engelmann's classic experiment, why were the oxygen-requiring bacteria clustered around the regions of Spirogyra algae that were bathed in red, blue, and violet light?

A) Chlorophyll is green and thus reflects green light.
B) The bacteria were immobile and that is where Engelmann happened to place them.
C) Photosynthesis is most active in those wavelengths of light and thus more oxygen is consumed by the algae in those regions.
D) Photosynthesis is most active in those wavelengths of light and thus more oxygen is produced by the algae in those regions.
E) The most energy-rich wavelengths of light are found at the edges of visible and nonvisible light (ultraviolet and infrared) .