Final answer:
The preposition 'à travers' should be replaced with 'pendant'.
Step-by-step explanation:
In the paragraph, the preposition à travers is incorrect. It should be replaced with the preposition pendant.
The corrected sentence would be: 'Il en rêve et a organisé ce circuit pendant dix ans.'
The preposition pendant is used to express a duration of time. In this context, it conveys that Alex organized the circuit 'for' or 'during' ten years.
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