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Actually, the Oort cloud contains massive amounts of cometary material, and I'd suspect the odds are not great that we actually have a reliable notion of what ratios of heavy water they contain, since we haven't sampled much, and their sources seem likely to have been diverse. If there's someplace in the solar system where water is in abundance, then odds are, that's where our water came from. I'm betting on the steady accrection theory--the water's here because it is steadily (or unsteadily) renewed from cometary material.