How do you detect the centre of multi-engine UAS with an odd number of engines?

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Question EASA Drone Quiz - Flight performance and planning

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Dense, uniform, horizontally extensive masses from which falls a continuous precipitation

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Large, dark, roundish clusters, often grouped together to form large banks at horizontal trend

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Ambient temperature

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None of the answers is correct

Id 105 - EASA Drone Quiz – Drone Pilot Certification - How do you detect the centre of multi-engine UAS with an odd number of engines?