题干

近年我国部分地区出现的手足口病主要由肠道EV71病毒感染而引起的一种传染病,下列说法正确的是

A:EV71病毒感染力很强,人体的第一、二道防线对它不起作用

B:EV71病毒只能刺激人体的B细胞产生抗体

C:防止EV71病毒的传播和蔓延最好的方法是对易感儿童注射抗体

D:某儿童感染一种EV71病毒后产生的抗体可能识别其他变种病毒

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D

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    The insects are important to farmers. When a honeybee lands on a flower in plant, pollen (花粉) sticks to its legs. When the bee lands on another flower, some of the pollen falls off and fertilizes the second plant.

    The act of spreading pollen is responsible for many fruits, vegetables, nuts and other crops. Yet about 30 percent of honeybees in the United States and other areas have died in recent years.

    Mike Leggett is a researcher at the University of Maryland. He wants to learn why so many bees are dying. “The number of colonies (蜂群) that die every winter has been one in three. So on average 30 percent of the colonies have died every winter over the last six winters. And that's a huge number.” His research team examined the pollen that honeybees carried to their homes. They found that the pollen contained high levels of 35 different pesticides (杀虫剂), which are chemicals used to protect plants. “Pesticides are used, and have been used, pretty widely, for centuries, to protect plants from diseases,” says Mr Leggett.

    Keith Oh linger, a farmer and beekeeper in Maryland, has watched many of his bees die every winter. Mr Ohlinger thinks widespread bee death is caused by several things happening at once. But he does not feel sure that pesticides are a part of the problem, “I felt it was the result of a lot of little things. I didn't feel that there was probably one smoking gun. But there's a division (分歧) there, some people feel that it is just one thing.”

    Honeybees are important to agriculture. This makes the search for an answer to their death very important for Mr Leggett's team. As he knows, one in every three bites of food we eat is somehow pollinated by honeybees.