As people's lives and work gradually enter high-rise buildings, people have less access to sunlight. Studies have found that sunlight is a necessary condition for the synthesis of vitamin D, and vitamin D is a key factor to promote calcium absorption. Some people have concluded that people are more susceptible to diseases caused by calcium deficiency, such as arm porosity. Which of the following is true and the above conclusions can be most questioned? ['Patients with osteoporosis can be relieved or cured by exposure to the sun.', 'Calcium-containing foods in modern diets are much richer than before.', 'Oral vitamin D tablets are a synthetic formula added with absorption enhancers.', 'Patients with osteoporosis are exposed to sunlight for the same length of time as others.']