Three. (Full marks 25 points) The product of $n$ consecutive natural numbers from 1 to $n$ is called the factorial of $n$, denoted as $n$! (For example, 5! $=5 \times 4 \times 3 \times 2 \times 1$). How many consecutive zeros are at the end of 1999! ? Explain your reasoning.

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Note: The original text uses "畍居" which seems to be a typo or an incorrect character. The correct term should be "reasoning" or "justification."