被动语态翻译练习

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As oil is found deep in the ground, its presence cannot be determined by a study of the surface. Consequently, a geological survey of the underground rock structure must be carried out. If it is thought that the rocks in a certain area contain oil, a “drilling rig” is assembled. The most obvious part of a drilling rig is called “a derrick”. It is used to lift sections of pipe, which are lowered into the hole made by the drilled. As the hole is being drilled, a steel pipe is pushed down to prevent the sides from falling in. If oil is struck, a cover is firmly fixed to the top of the pipe and the oil is allowed to escape through a series of values.

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