I'm not from that school but I can help you.
Answer:
a
because the mechanical wave is when it goes over and over again
The size of the force varies inversely as the square of the distance between the two charges. Therefore, if the distance between the two charges is doubled, the attraction or repulsion becomes weaker, decreasing to one-fourth of the original value.
These applications DO NOT INVOLVE harmful ionizing energy:
- MRI
- ultrasound
- laser surgery