Answer Happy

I struggle with understanding this one. When the source i moving
closer, the percieved wavelenght should get smaller, and therefore
the frequency higher. However that is, for some reason, incorrect.
What is the logic behind it?
29. On approaching a stationary observer, a train sounds its horn and decelerates at a constant rate. At time t the train passes by the observer and continues to decelerate at the same rate. Which diagram shows the variation with time of the frequency of the sound measured by the observer? A. frequency measured by observer B. frequency measured by observer horn's frequency horn's frequency time time C. frequency measured by observer D. frequency measured by observer horn's frequency horn's frequency t time t time