Which of the following statements is true?

A) Only the difference threshold varies from person to person.
B) Only the absolute threshold is constant for all people.
C) Both the difference threshold and the absolute threshold are constant for all people at all times.
D) Both the absolute threshold and the difference threshold vary from person to person over time.

The absolute threshold refers to the minimum amount of stimulus energy that can be detected by a person's sensory system, while the difference threshold refers to the minimum difference in stimulus intensity needed to detect a difference between two stimuli. Both of these thresholds can vary from person to person and can also change over time due to factors such as age, fatigue, and sensory adaptation. Therefore, statement D is the only true statement.