Velocity of sound in vacuum is .... $ms^{-1}$

$A$ hospital uses an ultrasonic scanner to locate tumours in a tissue. What is the wavelength of sound in the tissue in which the speed of sound is $1.7 \; km/s$? The operating frequency of the scanner is $4.2 \; MHz$.

Which of the following cannot be polarised?

Which one of the following statements is true?

If $V_m$ is the velocity of sound in moist air and $V_d$ is the velocity of sound in dry air,under identical conditions of pressure and temperature,which of the following is true?

$A$ sound wave passing through an ideal gas at $NTP$ produces a pressure change of $0.001 \ dyne/cm^2$ during adiabatic compression. The corresponding change in temperature $(\gamma = 1.5$ for the gas and atmospheric pressure is $1.013 \times 10^6 \ dyne/cm^2)$ is: