Final answer:
Criminals, mentally ill individuals, and terrorist groups pose threats to civil aviation.
Explanation:
Criminals, mentally ill individuals, and terrorist groups are three groups of people that may pose a threat to civil aviation. Criminals may attempt to smuggle weapons or engage in unlawful activities onboard planes, while mentally ill individuals may pose a risk due to unpredictable behavior. Terrorist groups aim to use aviation as a target for attacks, leading to increased security measures at airports post-9/11.
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