Scientists studied 76 patients with heart disease. They used "one volunteer to bring a dog to visit the patient", "one volunteer to visit the patient" and "no volunteer" to test them separately. The results of these patients found that the patient's anxiety level decreased by 24% in the first case, the patient's anxiety level decreased by only 10% in the second case, and the patient's anxiety level remained at the original level in the third case. So scientists believe that dogs can help people with heart disease reduce anxiety. Which of the following can most weaken a scientist's argument? ['Experiments with and without dogs were selected at two different time periods.', 'Most of the volunteers who bring dogs like and keep pet dogs by themselves.', 'Of the patients visited, the vast majority liked and raised their own pet dogs.', 'Most dogs taken by volunteers to visit patients are more meek.']