Someone builds an active replication system where there is only one front$-$end, and the front$-$end for each operation $($read or write$)$ forwards only one operation at a time to all the replicas and waits for all of the replicas to respond to the coordinator before sending back an answer to the client. For a read operation, the front$-$end sends back the highest timestamped value. Which consistency model does this system implement? $($Select the most appropriate answer.