As a scientist, Carl is concerned with the intensity of lights in the modern human environment. In his research project, Group A consists of volunteers who have agreed to sleep each night for a week with a 150 watt light bulb hanging over their bed. Group B will sleep in total darkness, and Group C will sleep in their usual environment. Alertness, blood pressure, respiration rate, and muscle tension will be sampled in the morning and evening. Carl is studying:

a. light perception.

b. the relationship between mood and sleep.

c. gerontology.

d. psychophysics.

I think it is either a or d.

psychophysis