A train moving with constant speed towards an observer on a nearby bridge sounds its horn, which emits a sound of frequency f.
As the train approaches the observer, which of the following statements is correct?
a. The observer hears a sound of frequency smaller than f and with a constant intensity.
b. The observer hears a sound of frequency smaller than f and with an increasing intensity.
c. The observer hears a sound of frequency larger than f and with a constant intensity.
d. The observer hears a sound of frequency larger than f and with an increasing intensity.