For ethical practices, the following two options are acceptable for Carl to post:
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A positive review of a tool he used in creating his invention - This is ethical as long as he is honest and transparent about his experience with the tool.
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A negative review of a rival product he has not used - This is not ethical, as posting a review of a product he has not personally used misrepresents his experience and could be misleading to consumers.
Therefore, only the first option is ethical for Carl to post regarding reviews.