A source of sound moves towards a stationary observer. Is the increase in pitch due to:

A source of sound moves towards a stationary observer. Is the increase in pitch due to:
(i) increase in the velocity of sound ?
(ii) actual or apparent change in wavelength ?
(iii) both ?

When a source of sound moves towards a stationary observer the increase in pitch is due to actual or apparent change in weavelength. It is so because due to motion of the source of sound towards observer at rest, waves get compressed as the effective velocity of sound waves relative to source becomes less than the actual. As a result of it, the wavelength of sound waves decreases and hence the observed pitch increases.