a为声源，发出声波，b为接收者，接收a发出的声波。a、b若运动，只限于沿两者连线
题型：不定项选择
问题：
a为声源，发出声波，b为接收者，接收a发出的声波。a、b若运动，只限于沿两者连线方向上。下列说法中正确的是[ ]
A．a静止，b向a运动，则b收到的声频比a发出的高
B．a、b向同一方向运动，则b收到的声频一定比a发出的高
C．a、b向同一方向运动，则b收到的声频一定比a发出的低
D．a、b都向相互背离的方向运动，则b收到的声频比a发出的高
题型：不定项选择
根据对话，选择与内容相符的图片。
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B.
C.
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Mike: I often do my homework.[ ]
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John: He's strong and active.[ ]
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B.
C.
5. John: What's your favourite fruit?
Amy: Grape is my favourite fruit.[ ]
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C.
