设声波在媒质中的传播速度为u，声源的频率为υS。若声源S不动，而接收器R相对于媒质以速度vR沿着S、R连线向着声源S运动，则位于S、R连线中点的质点P的振动频率为（　　）。

A、<img width="22" height="18" src="http://admintk.100xuexi.com/UpLoadImage/2013-05-07/37508b40-7910-4470-914e-0e1f4884c3bc.png" />

B、<img width="67" height="40" src="http://admintk.100xuexi.com/UpLoadImage/2013-05-07/30e8d713-f371-4fcd-97f9-a081d85692f9.png" />

C、<img width="70" height="39" src="http://admintk.100xuexi.com/UpLoadImage/2013-05-07/9eb00271-5b7f-4148-85d9-dd89f9e2a325.png" />

D、<img width="67" height="42" src="http://admintk.100xuexi.com/UpLoadImage/2013-05-07/f990436a-2dbf-4e40-871e-0838423dc4ab.png" />