Which revolutionary product innovation, first deployed effectively during WWII, allowed Allied forces to detect German U-boats at night and in poor weather conditions?

Throughout military history, technological innovations have dramatically changed warfare. Many military products we now take for granted were revolutionary when first introduced, transforming combat strategy and sometimes changing the course of history. This poll tests your knowledge about a significant product innovation that revolutionized naval warfare during a critical period of military history. Do you know which technology gave a decisive advantage to its developers?

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ASDIC/SONAR (underwater sound detection)
H2S airborne radar system
Enigma code-breaking computers
Magnetic Anomaly Detection (MAD) gear