A sound wave travels from the left to the right. The graph shows the variation of the displacement to the right of particles in the sound wave with distance, at one instant. [Diagram shows a displacement-distance graph for a sound wave. Displacement to the right is on the y-axis, distance on the x-axis. The wave starts at 0, goes up to a peak (A), crosses 0, goes down to a trough (B), crosses 0, goes up to a peak (D), etc. C is at a zero displacement point between A and B.] Which letter represents the centre of a compression?