单项选择题

某公司计量测试中心对某铁路隧道出口段混凝土衬砌进行雷达检测,检测里程及测线布置:DK371+318.0～DK371+783.0（洞口）,共465m。地质雷达检测隧道衬砌质量时,测试中心工作人员需要掌握以下基本知识。（3）若衬砌背部回填不密实,地质雷达数据有（）的特征。

A.反射信号弱，图像均匀且反射界面不明显
B.反射信号强，图像变化杂乱
C.反射信号强，图像分散的月牙状
D.反射信号弱，图像不均

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单项选择题

The sea is very big. Look at a map of the world. There is less land than sea. The sea covers three quarters of the world.The sea is very deep in some places. There is one spot, near Japan, where the sea is nearly 11 kilometres deep. The highest mountain in the world is about 9 kilometres high. If that mountain were put into the sea at that place, there would be 2 kilometres of water above it.If you have swum in the sea, you know that it is salty. You can taste the salt. Rivers, which flow into the sea, carry salt from the land into the sea. Some parts of the sea are less salty than other parts. There is one sea, called the Dead Sea, which is very salty. Because it is very salty, swimmes cannot sink! Fish cannot live in the Dead Sea.In most parts of the sea, there are plenty of fishes and plants. Some live near the top of the sea. Others live deep down. There are also millions of tiny living things that float in the sea. These floating things are very small. It is hard to see them. Many fish live by eating them.The sea can be very cold. Divers, who dive deep down in the sea, know this. On the top the water may be warm. When the diver goes downwards, the sea becomes colder and colder.Another thing happens. When the diver goes deeper, the water above presses down on him. It squeezes him. Then the diver has to wear clothes made of metal.Fish in the sea live ____.（）

A.nearthetopofthesea
B.deepdowninthesea
C.atdifferentdepthofthesea
D.ontinylivingthingsinthesea

单项选择题

A． B． C． D．

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