I'm from the south and I say Diet Pepsi- but only because that's what I want.
When I was growing up people always said cold drink but you have to say it as one word- coldrink. As in "would you like a coldrink? Yes, I'd love a diet pepsi." My dad's family in GA always says Coke but I think that's what they usually want for real. Or it might be- "would you like a coke? Yes, I'd love a sprite." Ohhh the confusing world of flavored carbonated beverages.
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"If you have men who will exclude any of God’s creatures from the shelter of compassion and pity, you will have men who will deal likewise with their fellow men." — St. Francis of Assisi, 1181-1226