1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution Slightly confused here. Since this is a displacement-time graph, is it correct to say that at X, since the displacement is zero that it can't be a point of compression because that would imply the air molecules are moving to and fro so they would be experiencing some displacement? If so, then all three points would be points of rarefaction (since at rarefaction the air molecules aren't moving thus displacement = 0) so the answer would be D?