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Two factors that affect human sleep patterns are the time of day and level of melatonin, a chemical found in one's body. For example, as the Sun goes down at night, the body knows it is time to go to sleep. Similarly, when a person's level of melatonin changes, they can become alert or sleepy. Using this information, how could these stimuli be described?

A. Hours of sunlight per day and melatonin levels are both Internal stimuli.

B. Hours of sunlight per day and melatonin levels are both external stimull.

C. Hours of sunlight per day is an external stimulus, while the level of melatonin in a person is an internal stimulus.

D. Hours of sunlight per day is an internal stimulus, while the level of melatonin in a person is an external stimulus.

E. Hours of sunlight per day and melatonin levels are both internal and external stimuli.​