Consider the following statements:

Statement - I: When an ultrasound travels from one medium to the other medium, its wavelength, frequency and speed change.

Statement - II: The frequency of an ultrasound wave remains almost unchanged while travelling from one medium to the other medium.

Statement - III: The frequency and speed of an ultrasound wave change when travelling from one medium to the other medium.

Statement - IV: 'Echocardiography’- a technique used in medical diagnostics employs ultrasound.

Which of the following combination of above statements are true?