Last week I told you about how the body uses vitamin C and foods that contain vitamin C, such as oranges and tomatoes. [22] Now I want to discuss vitamin D which is important to human nutrition because it helps the body to absorb calcium. Calcium as you know is essential for maintaining healthy bones and teeth. Vitamin D deficiency can lead to serious bone diseases. [23] So in the United States it is added to milk, a staple food in most people's diet. Vitamin D is sometimes called the sunshine vitamin. That's because the body requires sunlight in order to make use of it. Here's how it works: [ 24 ] when ultraviolet rays from the sun strike the vitamin D in our skin, the sunlight converts vitamin D to its active form and a substance called caiciferol is produced. Calciferol is what carries the calcium to our bones. [24] [25] If our bodies are not exposed to ultraviolet rays from the sun we cannot convert vitamin D to its active form, and thus cannot make use of any of the calcium that we have consumed in our food. Eventually we would suffer from a calcium deficiency and have weak bones. Luckily it takes only 10 to 20 minutes of exposure to sunlight a day to ensure sufficient vitamin D production.
Except in extremely atmospheric conditions like very heavy smog or a sunless climate, most people are able to get enough exposure to the sun. Doctors recommend that people who don't drink much milk take vitamin supplements and that everyone spend at least 10 minutes in the sun every day.
Why is vitamin D important?
【解题思路】
1．四项均以it开头。两项后接helps，一项接keeps，一项接prevents。
2．三项与人体健康有关，其中两项直接提及body。
结论：本题可能考查某种因素对人体健康的影响。应先辨别it的指代内容。
【解析】题干问为什么维生素D非常重要，我们可以在短文开头处找到线索。讲话者提到： “Now I want to discuss Vitamin D which is important to human nutrition because it helps the body to absorb calcium．(现在我要讲的是对人体健康很重要的维生素D，因为它能够帮助人体吸收钙。)”故选C。