The relationship between the speed, the frequency and the wavelength of a wave is given by:

where
f is the frequency
v is the speed of the wave

is its wavelength
for the pressure wave in this problem,

and v=1400 m/s, therefore its frequency is
That would be c. :) :) :)
Answer:
Newton's second law of motion states that the acceleration of a system is directly proportional to and in the same direction as the net external force acting on the system, and inversely proportional to its mass. If the only force acting on an object is due to gravity, the object is in free fall.
I believe the answer is heat.