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?