A service provider charges c dollars for the first 50 hours of service used per month and 40 cents for each 30 minutes in excess of 50 hours used during the month.If x is an integer greater than 50,which of the following expressions gives this service provider's charge,in dollars,for a month in which x hours of service were used?
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Ac+0.40x
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Bc+0.80x
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Cc+0.40（x-50）
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Dc+0.80（x-50）
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Ec+0.40（2x-50）
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正确答案: D