7．如图4所示为一列沿x轴正方向传播的简谐横波在t=0 时的波形。当某时刻S处的质点振动情况与t=0时R 处的质点振动情况相同时，关于P、R 处的质点，下列说法正确的是                                                                                                                                                                                                                                                                                                                                    

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