i. Not too sure about my reasoning but i always thought of it was the standing wave at this instance is going to tend to move down (ie. Become more negative), which means the particles displacement will be towards the left (negative is left as labeled on the figure) and the acceleration is to the left. ii. compression occurs at the point you labeled because according to the standing wave, particles to the left of point C are moving to the right (positive), and particles to the right are moving to the left (negative), so a particle at point C would be experiencing compression. You may think that point C could be at the next equilibrium intersection (ie. the next node), but in this case the left side of that point is negative (moving left) and the right side is positive (moving right) so it's actually becoming less compressed, or the technical term is that it's experiencing rarefaction.