thankyou 
ok same paper Q23
at Q, displacement can not be always zero as it is not a stationary wave, therefore we eliminate B, at R, its max. displacement and therefore it stopped so that it will then go back upwards, and hence we eliminate C now
as For point P, just like R its coming to rest, and therefore it should have MINIMUM speed and not MAXIMUM speed, . and therefore we eliminate A and the answer is D
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the explanation for the answer, point S is going to start going downwards now and its going to have maximum acceleration initially then slow down as it arrives at the negative amplitude
hope this is clear
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