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【多选题】
The main stages in aerodynamic design method and process of aircraft include: ____.
A.
The investigation and demonstration
B.
The concept and preliminary design
C.
The wind tunnel test and verification
D.
The flight tests
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参考解析：
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【单选题】The turbulent drag is ____ to the laminar drag.
A.
2 to 4 times
B.
3 to 5 times
C.
4 to 6 times
D.
5 to 7 times