$A$ train, standing in a station-yard, blows a whistle of frequency $400\,Hz$ in still air. The wind starts blowing in the direction from the yard to the station with a speed of $10\,ms^{-1}$. What will be the frequency and speed of sound for an observer? (The speed of sound in still air is $340\,ms^{-1}$)

• A
  $400\,Hz, 340\,ms^{-1}$
• B
  $ > 400\,Hz, 340\,ms^{-1}$
• C
  $400\,Hz, 350\,ms^{-1}$
• D
  $ < 400\,Hz, 330\,ms^{-1}$