Fluid Dynamics: Please Help!

Choose one:

We know that all gases are compressible fluids. Gas flows

$($a$)$ can be considered incompressible if the velocity of the flow is less than $30\%$ of the speed of sound

within the air.

$($b$)$ can be considered incompressible if the velocity of the flow is less than the speed of sound within

the flow of interest.

$($c$)$ can be regarded as incompressible if the velocity of the flow is less than $30\%$ of the speed of sound

within the gas of interest.

$($d$)$ are always considered compressible regardless of velocity.

$($e$)$ can never be considered compressible.