车站计算机和票房售票机最少可以保存（）的运营数据，闸机和自动售票机最少可以保存3天的运营数据。
A.15
B.25
C.20
D.7天
参考答案：D
I think dogs are good enough for _____ to take care of. [ ]
A. a 80-year-old grandmother
B. a 80-years -old grandmother
C. an 80-year-old grandmother
D. an 80-years -old grandmother
有木球、铝球、铜球各一个，它们在水中静止时的情况如图所示，则______球一定是空心的．
