题干

下列有关免疫的叙述,正确的是(  )

A:人体的前两道防线不具有特异性

B:抗体主要分布在血清中,所以人体被病菌侵染后只要注射血清就可以治疗疾病

C:艾滋病病毒侵入人体后主要破坏B淋巴细胞,从而破坏人体的免疫系统

D:吞噬细胞的吞噬作用没有特异性,因此只在非特异性免疫过程中起作用

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答案(点此获取答案解析)

A

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    When we think of food farming we usually think of green fields in the country, outside the city. Sam Chee works on a very different type of farm, in the center of the Canadian city of Toronto. “We grow fruit, vegetables and herbs,” says Sam. “But not many people know the farm is here. You can't see it from the street. We're on top of an office building, 50 meters above the ground.”
    Rooftop farming is becoming popular in cities like Toronto, New York and London. The roofs of buildings are often just wasted space, but this space can be used for growing good fresh food, right in the middle of the city. Sam explains how it works. “We grow everything in big boxes—there are hundreds of them. We catch rainwater to water the plants when the weather is dry.”
    The rooftop farm where Sam works belongs to a restaurant called Green Garden, on the ground floor of the building. The farm produces about half the restaurant's fruit and vegetables. Sam believes that the rooftop farm is what makes Green Garden special. “We help to keep food production local. We don't need to transport this food from anywhere and that saves energy. It's better for the environment.”
    The food from the rooftop is fresher too. “When the fruit and vegetables are ripe we can pick them and use them right away. They taste better when they're really fresh,” says Sam. “I hope that there will be more rooftop farms in the city soon. It could be the farming of the future.”