Here the main point is heart attacks and their influences over the health of elderly women. There are two consequences here: vision loss and risk to develop dementia. - this is the main point.
One conclusion is - Thus, demented elderly women are much more likely to suffer from vision loss than are other elderly women - Here you have like a Venn Diagramm, intersection being demented elderly women who suffered hear attacks. Boldfaced conclusion - steps taken to prevent heart attacks, such as dieting, may also prevent dementia
- is about how to prevent the dementia. is it relevant?! No! If it still seems relevant for you just read the argument and skip the boldfaced sentence. It does not affecе the conclusion.
Hence it is E.
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