2. 外语
  3. AIn the liquid-fuel rocket, the purpose of the valves Bis control the flow of the propellants, enabling the engine Cto work at d

AIn the liquid-fuel rocket, the purpose of the valves Bis control the flow of the propellants, enabling the engine Cto work at d

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问题 AIn the liquid-fuel rocket, the purpose of the valves Bis control the flow of the propellants, enabling the engine Cto work at different degrees Dof power.
选项 A、 
B、 
C、 
D、 
答案B
解析 Is control is an incorrect structure. An infinitive is needed after the purpose of the valves is. Correction: is to control.
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