Pregnant women are prone to symptoms of vitamin deficiency. Some people think that this is not caused by a lack of vitamins in the diet, but usually due to the large demand for vitamins when the baby in the abdomen grows. In order to evaluate the accuracy of the above conclusions, which of the following operations is most important? ['Test the daily diet of a pregnant woman who lacks vitamins to determine the vitamin content.', 'Test the daily diet of a pregnant woman who does not lack vitamins to determine the vitamin content.', 'Research on scientific recipes for pregnant women to determine the best recipes for pregnant women to get enough vitamins.', 'Test a pregnant woman and a non-pregnant woman who have sufficient vitamins in their daily diet and determine whether they are deficient in vitamins.']