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答案：D
解析：
Machine X, working at a constant rate, can produce x bolts per hour, and machine Y, working at a constant rate, can produce x+6 bolts per hour. Therefore, in one hour, both machines working together at their respective rates can produce x+x+6 equals (2x)+6 bolts. It follows that in 4 hours, both machines working together at their respective rates can produce 4((2x)+6) equals (8x)+24 bolts.
