They used interchangeable parts. This made the mass production possible and caused it to happen.
Answer: Oil.
Japan wanted southern Indochina's supply of oil as they were starved for it and unable to maintain their military. On the same day they attacked Pearl Harbour they also attacked the Philippines to gain access to this oil. The attack on Pearl Harbour was to prevent any armada of US ships from retaking the Philippines, which they succeeded at doing.
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The answer is D. The union won despite many casualties
