Electronic warfare sensors alone may not be the best way to identify threats because

The correct answer is that "Many Nations utilize commercial radar on their naval ships, which may lead to misidentification". 

Electronic Warfare Sensors alone are not enough to identify threats efficiently, if a ship misidentifies another nation's naval ship, it may result into a war or an argument between the two nations, which is why Electronic Warfare Sensors alone are not efficient enough to detect threats,

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