12．Scientists have discovered thirteen kinds of vitamins．They say vitamins help to carry out chemical changes within cells．If we do not get enough of the vitamins we need in our food，we are at risk of developing a number of diseases．Which foods should be eaten to keep us healthy？Let us look at some important vitamins．
Vitamin A helps prevent skin and other tissues from becoming dry．People who do not get enough vitamin A cannot see well in darkness．Vitamin A is found in fish liver oil and the yellow part of eggs．
Vitamin B-1is also called thiamine．Thiamine changes starchy（含淀粉的）foods into energy．Thiamine is found not just in whole grains like brown rice，but also in beans and peas，nuts，and meat and fish．
Vitamin B-12is needed so folic acid can do its work．Together，they help produce red blood cells．Folic acid has been shown to prevent physical problems in babies when taken by their mothers during pregnancy．Vitamin B-12is found naturally in foods like eggs，meat，fish and milk products．Vitamin B-12is found in green leafy vegetables and other foods，like legumes and citrus fruits （柑橘類水果）．
Vitamin C is needed for strong bones and teeth．The body stores little vitamin C．So we must get it every day in foods such as citrus fruits，tomatoes and uncooked cabbage．
Vitamin D prevents the children's bone disease rickets （佝僂�。甎ltraviolet light from the sun changes a substance in the skin into vitamin D．Fish liver oil also contains vitamin D．
Vitamin K is needed for healthy blood．It thickens the blood around a cut to stop bleeding．It can also be found in pork products，liver and in vegetables like cabbage，kale and spinach （菠菜）．
Vitamins are important to our health．A lack of required vitamins can lead to health problems．
28．What is the main idea of the passage？D
A．There are thirteen kinds of vitamins in total．
B．One can not live without enough vitamins．
C．Vitamins can carry out chemical changes within cells．
D．Enough vitamins are vital to keep healthy．
29．Women who wish to become mothers should take inC．
A．vitamin A
B．vitamin B-1
C．vitamin B-12
D．vitamin C
30．What can change a substance in the skin into vitamin D？B
A．Fish liver oil．
B．Ultraviolet light from the sun．
C．Fresh milk drunk by people．
D．Doing a certain amount of exercise．
31．According to the passage，taking in enough vitamin C canC．
A．make our eyesight better
B．help to stop bleeding
C．make our bones and teeth strong
D．help produce red blood cells．
分析 本文屬于說明文閱讀，作者通過這篇文章向我們們介紹了幾種不同的維生素，告訴我們這些維生素對人體的不同作用，并向我們說明足夠的維生素對保持健康是至關(guān)重要的．
解答 28．D 主旨大意題，通讀全文可知本文中作者向我們介紹了幾種維生素，并向我們說明足夠的維生素對保持健康是至關(guān)重要的，故選D．
29．C 細(xì)節(jié)理解題，根據(jù)第四段Vitamin B-12is needed so folic acid can do its work．Together，they help produce red blood cells．Folic acid has been shown to prevent physical problems in babies when taken by their mothers during pregnancy．可知維生素 B-12能幫助懷孕的母親產(chǎn)生紅血細(xì)胞，也能預(yù)防嬰兒的身體問題，因此希望成為母親的婦女應(yīng)該攝入維生素 B-12，故選C．
30．B 細(xì)節(jié)理解題，根據(jù)第六段Vitamin D prevents the children's bone disease rickets （佝僂�。甎ltraviolet light from the sun changes a substance in the skin into vitamin D．Fish liver oil also contains vitamin D．可知陽光中的紫外線可以使皮膚中的一種物質(zhì)轉(zhuǎn)變成維生素D，故選B．
31．C 推理判斷題，根據(jù)第五段Vitamin C is needed for strong bones and teeth．The body stores little vitamin C．So we must get it every day in foods such as citrus fruits，tomatoes and uncooked cabbage．可知維生素C可以使我們的骨骼和牙齒更強壯，故選C．
點評 考查學(xué)生的細(xì)節(jié)理解和推理判斷能力．做細(xì)節(jié)理解題時一定要找到文章中的原句，和題干進行比較，再做出正確選擇．在做推理判斷題時不要以個人的主觀想象代替文章的事實，要根據(jù)文章事實進行合乎邏輯的推理判斷．
