Which of the following waves cannot be polarized?

Which of the following waves is used in sonography?

The speed of sound in mercury at a certain temperature is $1450 \, m/s$. Given the density of mercury as $13.6 \times 10^3 \, kg/m^3$,the bulk modulus for mercury is:

Sound waves travel at $350 \text{ m/s}$ through warm air and at $3500 \text{ m/s}$ through brass. Which of the following is correct about the wavelength of a $700 \text{ Hz}$ acoustic wave as it enters brass from warm air?

$A$ person standing between two parallel cliffs fires a gun and hears two echoes,the first echo after $1 \ s$ and the second echo after $3 \ s$. If the velocity of sound is $340 \ m/s$,the distance between the two cliffs is: (in $m$)

$A$ mechanical wave (sound wave) in a gas is: