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?
(A) 7.2
(B) 7.3
(C) 7.5
(D) 7.6
(E) 7.7
(A) 7.2
(B) 7.3
(C) 7.5
(D) 7.6
(E) 7.7
【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
小时和