多普勒效应
参考答案：多普勒效应：由于声源和接收体之间的相对运动，引起超声波回声频率发生改变，这种频移现象称为多普勒效应。
一对渐开线直齿圆柱齿轮，只要压力角相等，就能保证正确啮合。( )
Mark Twain presented the 19th-century America in his own unique way. Discuss Twain’s art of fiction: the setting, the language, and the characters, etc., based on his novel Adventures of Huckleberry Finn.
