Yeah, thanks for that great, pithy explanation!
While we're here, I'll break this question down for anybody else who's interested.
The core is as follows:
P: unemployed retirees are also more prone to heart attacks on Mondays ---> C: there must be some reason other than back-to-work stress that makes Monday heart attack day.
The assumption here is subtle and not very far-fetched: that unemployed retirees do not still feel the stress associated with going back to work. (A) undermines this assumption by showing that these people are still beginning big projects, i.e., their work, on Mondays.
(B) is out as Timmy explained because we are only interested in people who are unemployed.
I would also add, in response to the original post, that it would NOT be okay to assume that people with part-time jobs are starting work on Mondays. Ask yourself: could it be otherwise? If the answer is yes, as it is here, then it's not a safe assumption.
(C) is out because dietary and health habits are totally out of scope.
(D) is incorrect because it doesn't have anything to do with why retirees are having heart attacks on Monday, specifically.
(E) is out for the same reason.
#officialexplanation