What helped me better understand this one (after missing it), is the question "...if true, most weakens" is analogous to considering, if you were defending someone who is a suspect of an investigation and the DA gave you 5 pieces of information considered evidence, which of the 5 would be most detrimental to your case? e.g., a) being seen with the victim the day before the event or b) the existence of a video showing your client committing the crime? Obviously, it would be B. Same rationale here