What does the passage mainly discuss?

Câu hỏi: Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 30 to 34.
Social support refers to the help and comfort that is provided by other people or groups. Having a good friend or a support group to talk with can be a key factor in dealing with stress in a positive way. For those individuals who like to keep pets, the companionship of dogs, cats and other animals has been found to be another significant form of social support.
In a study by Karen Allen, the findings confirmed that having a pet present significantly lowered the stress reaction in subjects who were required to complete a stressful talk. Allen performed the study using female subjects who owned dogs. The subjects were divided into random groups. In the first groups, the subjects were allowed to take a good friend with them into the room where the task would be done; in the second group, no support was provided in the room; in the third group, the subjects were allowed to take their dogs with them. During the task, which consisted of solving a very difficult math, a number of physiological reactions were measured, including blood pressure and heart rate. As expected, the presence of emotional support seemed to influence the level of stress. The group that was allowed to keep their dogs in the room showed significantly less tress as measured by their physiological reactions to the task whereas the control group without emotional support had a higher stress level.
What was somewhat unexpected in the study was the negative response by subjects to having a good friend in the room. Rather than decreasing the stress level, the presence of a human actually seemed to cause the highest stress reaction. A typical response by a subject was a pulse rate below 80 with the dog present, about 100 when alone in the room, and more than 115 when the human friend was seated nearby. The conclusion suggested that having a pet may be a more effective way to lower stress than having a human companion. At a minimum, Allen proved that pets do provide very positive emotional support under conditions of stress.
(Source: Practice exercises for the TOEFL by Barron's)
What does the passage mainly discuss?
A. The importance of friendship in releasing stress
B. The factor that makes stress unbearable
C. A summary of a study about pets
D. The role of pets in relieving stress