20世纪70年代以前，课程研究领域主要采用“质的研究”方法。
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采用隔爆外壳的隔爆型式通常称为隔爆型，用字母“D、”表示
A. 正确
B. 错误
Before starting an engine by air, the engine must（）
A. be turned by a turning gear for at least one minute
B. be turned by a turning gear for at least two minutes
C. be turned at least one complete turn
D. be turned at least two complete tur
材料的疲劳强度随着其抗拉强度增高而增高。
A. 正确
B. 错误
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