单选题 Among all the malignancies, lung cancer is the biggest killer; more than 100 000 Americans die of the disease. Giving up smoking is one of obvious ways to reduce the risk, but another answer may lie in the kitchen. According to a new report, even heavy smokers may be protected from developing lung cancer by eating a daily portion of carrots, spinach or any other vegetable or fruit containing a form of vitamin A called carotene. The finding, published in THE LANCET, is part of a long-range investigation of diet and disease. Since 1957, some American researchers have monitored the dietary habits and medical histories of 2 000 middle-aged men employed by the Western Electric Co. in Chicago. Led by Dr. Richard Shekelle of St. Luke's Medical Center, the researchers recently began to sort out the links between the subjects' dietary patterns and cancer. Other studies of animals and humans have suggested that vitamin A offer some protection against lung cancer. The correlation seemed logical, explains Shekelle, since vitamin A is essential for the growth of the epithelial tissue that lines the airways of the lungs. Vegetables: But the earlier research did not distinguish between the two different forms of the vitamin. "Preformed" vitamin A, known as retinol, is found mainly in liver and dairy products like milk, cheese, butter and eggs. But vitamin A is also made in the body from carotene, which is abundant in a variety of vegetables and fruits, including carrots, spinach, squash, tomatoes, sweet potatoes and apples. In the study, Shekelle and his colleagues found little correlation between the incidence of lung cancer and the consumption of foods containing preformed vitamin A. But when they examined the data on carotene intake, they discovered a significant relationship. Among the 488 men who had the lowest level of carotene consumption, there were 14 cases of lung cancer, in a group of the same size that ate the most carotene, only two cases developed. The apparent protective effect of carotene held up even for long-time smokers—but to a lesser degree. Further studies will be necessary before the link between lung cancer and carotene can be firmly established. In the meantime, researchers warn against taking large numbers of vitamin A pills, because the tablets contain a form of the chemical that can be extremely toxic in high doses. Instead, they advise a well-balanced diet that includes goods rich in carotene. For a smoker, a half-cup of carrots every day might possibly make the difference between life and death.

【正确答案】 C

【答案解析】解析：细节题。题目问的是哪一项中不可能发现维生素A?根据第四段最后两句“‘Preformed’vitamin A，known as retinol，is found mainly in liver and dairy products like milk，cheese，butter and eggs． But vitamin A is also made in the body from carotene，which is abundant in a variety of vegetables and fruits，including carrots，spinach，squash，tomatoes，sweet potatoes and apples”可知，维A能在肝制品和奶制品中找到，而胡萝卜素主要能在胡萝卜、菠菜、南瓜、西红柿、甘薯和苹果等蔬菜水果中找到。只有apple文中没有维A，故选C。