Multiple Choice, Single Answer

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Venice has long fought the sea, but its newest defence is a row of mobile barriers that normally rest unseen on the seabed. When forecasters predict an unusually high tide, compressed air is pumped into hollow steel gates, lifting them above the surface to seal the lagoon from the open Adriatic. Once the surge passes, the gates fill with water again and sink out of sight, leaving the entrance clear for ships. Engineers stress that the system is meant for exceptional tides, not everyday water levels, because raising the barriers too often would disrupt the port traffic the city depends on.