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问题 多选题

关于速度和加速度，以下说法中正确的是（　　）

A．速度为零，加速度也一定为零

B．速度变化，加速度也一定变化

C．加速度变大，速度有可能变小

D．速度变小，加速度有可能不变

答案

A、物体速度为零，加速度不一定为零，如自由落体初始位置，故A错误；

B、速度变化，加速度不一定变化，如匀加速直线运动，故B错误；

C、加速度变大，速度可能减小，如物体做加速度越来越大的减速运动，故C正确；

D、速度减小，加速度可能不变，如物体做匀减速直线运动，故D正确．

故选CD

单项选择题

About fifty years ago, plant physiologists set out to grow roots by themselves in solutions in laboratory flasks. The scientists found that the nutrition of isolated roots was quite simple. They required sugar and the usual minerals and vitamins. However, they did not require organic nitrogen compounds. These roots got along free on mineral inorganic nitrogen. Roots are capable of making their own proteins and other organic compounds. These activities by roots require energy, of course. The process of respiration uses sugar to make the high energy compound ATP, which drives the biochemical reactions. Respiration also requires oxygen. Highly active roots require a good deal of oxygen.
The study of isolated roots has provided an understanding of the relationship between shoots and roots in intact plants. The leaves of the shoots provide the roots with sugar and vitamins, and the roots provide the shoots with water and minerals. In addition, roots can provide the shoots withorganic nitrogen compounds. This comes in handy for the growth of buds in the early spring when leaves are not yet functioning. Once leaves begin photosynthesizing, they produce protein, but only mature leaves can "export" protein to the rest of the plant in the form of amino acids.

What is the main topic of the passage

A．The relationship between a plant’s roots and its shoots.

B．What can be learned by growing roots in isolation.

C．How plants can be grown without roots.

D．What elements are necessary for the growth of plants.
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问答题 简答题

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