Actually we don't need calculate much to answer the question.
We have decline from 9,680 to 9,600 (by 80 points) and then by 20%. Now, since 80 is less than 1% of 9,680, thus the overall decline would be slightly more than 20%. Only answer choice C fits.
P.S. General formula for percent increase or decrease, or percent change:
Again and again Bunuel rejoice my eyes with those eliminations.
What looks so huge as to be done in 20 sec and becomes as easy so that even I can clap it.
Please advise how to learn it - I mean elimination?