wave without a medium. series of compressions only. pattern of up and down vibrations. pattern of compressions and rarefactions.
I gave my students this multiple choice question: Sound travels as a

Only one answer is correct among these 4 choices:
- wave without a medium.
- series of compressions only.
- pattern of up and down vibrations.
- pattern of compressions and rarefactions.

Could you tell me which one is correct?

pattern of compressions and rarefactions.