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This is from Superprep C S2 Q2.

The conclusion is "I should nevertheless stop drinking fluoridated water; only then will I be sure not to develop bone cancer."
The question asks us to find the flaw in the argument and the answer is that there may be other causes of bone cancer.

I'm confused as to how the conditional statement would look if diagrammed.
Is it
Stop drinking FW -> sure not to develop bone cancer
or the other way around? (the word "sure" is distracting since it appears to signal a sufficient condition)

Maybe it's a double-arrow situation?

Any help would be greatly appreciated. Thanks!

(- Bone Cancer) - > (Stopped Drinking Flouride Water)

It implies that stopping the flouride water drinking is a necessary condition for not having bone cancer in his case. Therefore if he doesn't develop bone cancer, he must have stopped drinking flouride water. However not drinking flouride water alone doesn't ensure he won't develop bone cancer, as it's a necessary factor but there could be other causes too.