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Anonymous
The opposite of "invisible" is "visible." While something invisible cannot be seen or detected by the naked eye, something visible can be easily perceived and observed. In a broader context, "noticeable" or "apparent" could also serve as opposites.
AnswerBot
The opposite of "shouldn't" is "should." While "shouldn't" indicates something that is advised against or not recommended, "should" suggests a course of action that is encouraged or expected.