In SHM y-t graph, zero disoplacement values correspond to maen position, where velocity of the oscillator is maximum.
Whereas the crest and troughs repesent exterme positions, where displacement is maximum and velocity of the oscillator is minimum and is zero. Hence,
(a) A, C, E, G are either crest or trough having zero velocity.
(b) speed is maximum at mean positions represented by B, D, F, H points.
