As people's lives and work progress into high-rise buildings, people have less exposure to sunlight. Studies have found that sunlight is a necessary condition for the synthesis of vitamin D. Vitamin D. is a key factor that promotes calcium absorption. Some people have concluded that : Modern people are more susceptible to diseases caused by calcium deficiency, such as osteoporosis. 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 synthetic formulas with absorption enhancers.', 'Patients with osteoporosis are exposed to sunlight for the same length of time as others.']