A train has just complicated a U-curve in a track which is a semicircle. The engine is at the forward end of the semicircular part of the track while the last carriage is at the rear end of the semicircular track. The driver blows a whistle of frequency 200 Hz. Velocity of sound is 340 m/sec. Then the apparent frequency as observed by a passenger in the middle of a train when the speed of the train is 30 m/sec is

• A.            209 Hz                                     
• B.            288 Hz
• C.            200 Hz                                     
• D.            181 Hz