The research on the DDV is less at home and most of them only buy the foreign products or imitate them, and hasn't mastered the design theory and the design method.
而国内对套管阀的研究工作较少，仅有的套管阀研究工作只是对国外产品的结构仿制，完全没有掌握套管阀的设计理论和设计方法。
The research on the DDV is less at home and most of them only buy the foreign products or imitate them, and hasn't mastered the design theory and the design method.
而国内对套管阀的研究工作较少，仅有的套管阀研究工作只是对国外产品的结构仿制，完全没有掌握套管阀的设计理论和设计方法。
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