One can never tell whether another person is acting from an ulterior motive; therefore, it is impossible to tell whether someone's action is moral, and so one should action rather than its morality.
What this question is testing
Your task
Break the argument into its conclusion and evidence, then do exactly what the question stem asks with that structure.
Common trap
Answers that sound relevant to the topic but don't connect to the argument's actual reasoning.
Winning move
Predict what a right answer must do, then test each choice against the conclusion-evidence gap.
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