A recently completed study of several hundred subjects, all of approximately the same age, showed that those who exercised regularly during the study were much less likely to die during the study.
Conclusion
Exercise can actually increase one's life span.
Evidence
In a study of several hundred similar aged people, people who exercised regularly during the study were much less likely to die.
Evaluate
Because the evidence involves a Curious Comparison (people who were X were much less likely to be Y) and the evidence uses a Causal verb (X can actually increase Y), we can use our Curious Fact / Explanation mindset.
Curious Fact ... why was there a correlation between exercising regularly and living longer? Author's Explanation ... exercising is causing them to live longer
We want to consider Alternate Explanations and how plausible the Author's Explanation is. Whenever there's a correlation between two factors (in this case, regular exercise and living longer), authors will conclude one thing caused the other.
We need to consider Reverse Causality and Third Factor alternate explanations.
In this instance, Reverse Causality doesn't make any sense. We couldn't argue that not dying caused them to exercise regularly.
But it makes sense to argue there could be a Third Factor, such as lack of illness, that is creating the correlation.
People who are sick / enfeebled are both less likely to exercise regularly and more likely to die.
Goal
Since this is Strengthen, we need an answer choice to rule out an alternate explanation for the correlation, or increase the plausibility that exercise can increase life span.