Sales manager: The highest priority should be given to the needs of the sales department, because without successful sales whole would fail.
Shipping manager: There are several departments other than sales that also must function successfully for the company to succeed. It is impossible to to all of them.
What this question is testing
Sales manager's argument
Sales is critical to the company, so sales should get the highest priority.
Shipping manager's response
Hold on — by your logic, lots of departments deserve top priority, and that's a contradiction. You can't have multiple "highests."
Evaluate
The shipping manager isn't arguing that sales doesn't matter, or that some other department is more important. Instead, the move is to take the sales manager's implicit rule — — and run it through more cases. If sales is necessary so it gets top priority, then by the same logic so does shipping, finance, manufacturing, etc. But "highest priority" is a single slot. So the rule produces an impossible result. That's a classic reductio: showing the assumption leads somewhere absurd.
Goal
The right answer says the shipping manager points out the absurd consequence of the sales manager's assumption that necessity earns top priority.
Reading along? Open the full official question in LawHub — we show a fragment here and keep the reasoning in our own words.