One researcher found that people with low levels of the immune system scored lower on mental health tests than people with high levels of the immune system. The researcher concluded that the immune system can protect not only against physical illness but also mental illness. In fact, this causality is not valid. Which of the following is the most serious question? ["The researcher's theoretical research and the experiments performed are not consistent in time.", 'The human immune system is closely related to mental health.', 'Some people with a high immune system score differently on psychological tests than people with a normal immune system.', 'In general, intense stress first causes mental illness and then reduces the level of activity of the immune system.']