Newton's laws are not applicable in non-inertial frames.
These are frames that are under acceleration.
Newton's laws can be applied to these frames(though not common) by applying a special force called pseudo force that acts in the opposite direction of the acceleration.
Newton's laws terribly fail in explaining the quantum world.
Non-inertial frames it is!
Hope it helps!
A) because wave speed remains constant.
The wave speed of a wave in a medium depends on the nature of the medium.
For transverse waves in a string given by,
, T = tension , m = linear density
For longitudinal waves in a solid,
, ξ = modulus of elasticity, ρ = density
Like wise for different media, different properties and parameters govern the speed of waves moving through them.
It is important to mind in v = fλ ,f or λ does not influence on speed of wave in a medium but shows how f and λ varies as a wave propagate in a medium(/media).