关于正常菌群的叙述，正确的是（）。
A．一般情况下，正常菌群对人体有益无害
B．肠道内的双歧杆菌产生大量碱性物质，能拮抗肠道细菌感染
C．口腔中的正常菌群主要为需氧菌，少数为厌氧菌
D．即使是健康胎儿，也携带正常菌群
E．在人的一生中，正常菌群的种类和数量保持稳定
参考答案：A
解析：
正常菌群：是寄生在正常人体表及与外界相通的腔道中的微生物群体，正常情况下对人体有益无害。生物学意义：生物屏障作用，营养作用，抗肿瘤作用，免疫作用等。口腔中正常菌群主要为厌氧菌；随着人的衰老，正常菌群的种类和数量发生改变。
| 阅读理解。 根据短文内容，从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为 多余选项。 | |
| One of the favorite foods in the United States is the hamburger, a kind of flat round bread with fine-cut beef in between. 1 . At fast food restaurants, people order their food, wait a few minutes to get it over the counter, and carry it to their tables themselves. 2 .At some fast food restaurants , people can order their food, pay for it and pick it up without leaving their cars. 3 . Most of the restaurants sell hamburgers or other popular foods among Americans, In addition, there are many fast food restaurants that serve Chinese food, Italian food, chicken, seafood and ice-cream. The idea of a fast food restaurant is so popular that nearly every kind of food can be found in one. 4 . First , they are not formal restaurants. People wear any type of dress when they go to a fast food place. Second, they are fast. People who are busy do not want to spend time preparing their food or waiting while someone else prepares it. 5 . Finally , most food in a fast food restaurant is not expensive. Therefore, people can often eat at a fast food restaurant without spending too much money, while they may not be able to go to more expensive restaurants very often. | |
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