Critical Reasoning (CR) | Re: When exercising the muscles in one’s back, it is important

Good Afternoon Everyone
Special thanks toGMATNinja ,KB04 and Hasan for their help and explanation. I only have 1 more question -
How do we realize when to use necessary and sufficient point of reference and when to use simple negation. I hope my questions makes sense, because when seen as explained by GMATNINJA it makes sense but how to when to think on those lines, especially in exam when we all would only have few seconds?

So are there any markers we can look out for while