问题
选择题
在研究下列问题时，能把物体看成质点的是
A．研究旋转的乒乓球
B．研究一列火车通过长江大桥所需的时间
C．研究绕太阳运动的地球
D．表现精彩的芭蕾舞演员
答案
答案：C
物体的大小或形状对所研究的对象没有影响。ABD都有影响。所以答案是C
搜题请搜索小程序“爱下问搜题”
在研究下列问题时，能把物体看成质点的是
A．研究旋转的乒乓球
B．研究一列火车通过长江大桥所需的时间
C．研究绕太阳运动的地球
D．表现精彩的芭蕾舞演员
答案：C
物体的大小或形状对所研究的对象没有影响。ABD都有影响。所以答案是C
Choose the words or expressions and complete the passage (选择最恰当的单词或词语，完成短文) 1．We have always been warned of the dangers of the forest fires. They cause 1 to a forest. They kill many plants and animals in an eco-system, which is a group of living things and their environment. But have you ever heard of a forest fire that can be 2 for a forest? 2．People start some of the fires with special reasons to actually do well to the eco-system. 3．In fact, fires are a natural part of any eco-system. Without fires, the oldest and largest trees would stop sunlight from reaching the forest floor. Fires can 3 some of these old trees. When the trees die, they provide something good for the earth. New trees can grow strong and healthy in their place. 4．Some trees even need fires to grow. These trees hold their seeds(种子) inside. Heats from a fire allow the coat of the seeds to open up. Then the seeds fall to the ground and have the 4 to grow. 5．Scientists, who know about nature’s needs, start some of these fires, called controlled burns. A fire is started and kept under control. This way, the fire does not spread out of the area or 5 a large space of the forest. Fires can be dangerous for animals. Even when animals continue to live after forest fires, they can go hungry 6 their food has been damaged in the fire. Small, controlled fires mean that animals will not have to move far to find food. After some time, small plants begin to grow. Animals return soon after the plants return. This 7 makes an eco-system. 6．So the next time you hear about a forest fire, think about the good as well as the bad. It may take a new eco-system a while to return, but it will likely come back stronger and healthier than ever.
|