An observer is standing on a railway platform. A train passes the observer at constant speed while emitting sound of constant frequency 
from its whistle. What does the observer hear?
sound of a decreasing frequency as the train approaches and of an increasing frequency as it moves away
sound of a higher frequency than as the train approaches and of a lower frequency than as the train moves away
sound of a lower frequency than as the train approaches and of a higher frequency than as the train moves away
sound of an increasing frequency as the train approaches and as it moves away
