How would World War be fought if one ever breaks out?

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There are many who say there will be an exhibition of nuclear weapons. But I think there will be a wide use of bio weapons.Bioweapons are really dangerous, it takes toll of thousands of organisms including man. My theory is simple, if you want to create a nuclear bomb the cost is really high. But if you want to create a bioweapon it is really simple and doesn't need much investment. What is your opinion?