Historian: Political regimes that routinely censor various forms of expression on the grounds that they undermine public morality inevitably attempt to expand the categories of proscribed expression to include criticisms that these regimes perceive to threaten their power.
Your task
Find the assumption the argument requires in order for its conclusion to hold.
Common trap
Answers that would help the argument but aren't strictly required (sufficient, not necessary).
Winning move
Negate each choice — the right one breaks the argument when negated.