Thanks for your response!
It's a subtle distinction, but the issue that you are discussing is that of how the 
choice to be sedentary was made. Whether that choice was based on being poorly informed, lack of good advice, love of tv, bad decision-making, or whatever else, if something has led people to 
choose to be sedentary, that does not weaken the author's conclusion about the choice to be sedentary:
some cause 
 choice
 choice  
 sedentary life  

  societal detriment
Do you see how what 
led to that 
choice is not really relevant?  
The correct answer choice, meanwhile, provides a completely different cause--physical conditions that lead to a predisposition to a sedentary lifestyle--this calls into question the author's conclusion about the detrimental effects caused by people's 
choosing to be sedentary:
Physical predisposition  

  sedentary life  

  societal detriment.
I hope that's helpful!  Please let me know whether this clears that one up--thanks!
~Steve