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Optimistic Rollups are solutions that bundle sidechain or off-chain transactions into a single transaction that is then committed to L1. Rollups use fraud-proofs to ensure that all transactions are legitimate. L2 rollup sidechains are responsible for verification and contract execution, while the L1 exclusively stores immutable transaction data. This significantly reduces the gas costs compared to ZK-rollups, and opens the door to the ability of achieving x10-x100 improvements in the transaction throughput.