I was between B C and D in this question and chose B because i thought it would strengthen the idea that social problems caused the decline. If the dry spell caused the social problems then reinforcing the notion that the social problems did in face cause the decline seemed to be a good strengthener.
I decided against D for the same reasons i used to pick B... because social conditions were deteriorating (i.e. the roads were worsening there was sickness being passed around like a plague) that would account for the decline and not a drought or social problems.
Finally, I didnt choose C because of the word "sudden declines" i the passage it says cooling caused "extended droughts" and that these droughts caused the social upheaval. inherent in this is the idea of time, all the these things eventually added up to the toppling of the kingdom BUT it did not happen overnight. Can someone please help clear this up?