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题目材料：

The "Oxford comma," also known as the "serial comma," is a comma used before a coordinating conjunction before the last item in a list of three or more. For example, in "the bus arrives on the hour and every fifteen, thirty, and forty-five minutes after the hour," the Oxford comma may be seen following thirty; without the Oxford comma, the phrase would be "the bus arrives on the hour and every fifteen, thirty and forty-five minutes after the hour." There are no definitive grammatical rulings on when it should be used and by whom, although journalists rarely make use of it. While it is not always necessary, it can prevent otherwise ambiguous statements (for example: "they hired the two brothers, David and James," in which they hired two men, versus "they hired the two brothers, David, and James," in which they hired two brothers plus David and James, four in total). Therefore, writers should be aware of its use even if they abstain from it in unambiguous cases.