问题
选择题
My sister has learnt English . [ ]
A. for twelve years ago
B. since she was four
C. twelve years ago
D. at the age of four
答案
答案：B
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My sister has learnt English . [ ]
A. for twelve years ago
B. since she was four
C. twelve years ago
D. at the age of four
答案：B
下表记录的是小刚同学40m跑的有关数据，请做有关计算：
| 路程/m | 10 | 20 | 30 | 40 |
| 时间/s | 2.5 | 4.5 | 6.2 | 7.7 |
（2）后20m内的平均速度是______m/s；
（3）全程的平均速度为______m/s；
（4）小刚在第______个10m内运动最快．