A、<img width="29" height="19" src="http://admintk.100xuexi.com/UpLoadImage/2013-05-07/e48d8e85-c191-4cca-90cc-ceec162b2fba.png" />
B、<img width="37" height="37" src="http://admintk.100xuexi.com/UpLoadImage/2013-05-07/01f3f235-7415-4fc1-81a5-a545aa25f3ac.png" />
C、<img width="22" height="18" src="http://admintk.100xuexi.com/UpLoadImage/2013-05-07/a4e537cb-dde4-4963-973f-281616a5e0ea.png" />
D、<img width="32" height="34" src="http://admintk.100xuexi.com/UpLoadImage/2013-05-07/07d061d0-e5aa-4778-8552-2fc9b6254d9c.png" />
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，当观察者迎着波源运动时，他接收到的声波频率是（ ）。[2010年真题]