Setting a specific Block Number is predictable because each block takes ~13sec (approximately) to mine, so why was Terminal Total Difficulty (TTD) chosen to trigger the Merge?

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Because in the PoW network, the canonical chain represents the chain with the highest amount of work, not highest block number. If we use the chain with the highest block number, then a minority chain with a lower hash power can reach a certain block number faster and hijack the transition to the merge upgrade. That is why it's important for the PoS chain to pick the PoW chain with the highest amount of work, and that is determined by the total terminal difficulty, or TTD.

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