单选题 There are 4 card-processing machines in an office. The fastest of these machines processes x cards in 7 hours and the slowest processes x cards in 8 hours. Which of the following could NOT be the average time per machine for each of the 4 machines to process x cards?

【正确答案】 A
【答案解析】[解析] 根据题意可知，其余两台机器处理z张卡片所用的时间t一定为：7≤t≤8。由此可知四台机器每台都处理x张卡片的平均时间的最小值和最大值分别为：[*]小时和[*]小时
因此7.2落在了平均值之外，不可能是这四台机器每台都处理x张卡片所用的平均时间。