问题
不定项选择
某人爬山，从山脚爬上山顶，然后又从原路返回到山脚，上山的平均速率为v1，下山的平均速率为v2，则往返的平均速度的大小和平均速率是[ ]
A、
，
B、
，
C、0，
D、0，
答案
答案：D
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某人爬山，从山脚爬上山顶，然后又从原路返回到山脚，上山的平均速率为v1，下山的平均速率为v2，则往返的平均速度的大小和平均速率是[ ]
A、，
B、，
C、0，
D、0，
答案：D
| 完型填空 。 | |||
| Do you like bubble gum (泡泡糖)? Do you know when it was 1 ? In the 1870s, an American discovered bubble gum. 2 name was Thomas Adams. He wanted to find the use of a chicle（一种树胶). Chicle is a Spanish 3 for sticky water that comes from one kind of Mexican tree. Mr Adams wanted to make rubber 4 chicle. He worked in his home while he tried to find a way to make the chicle become stronger. He had a son 5 name was Horatio. He also helped his father now and then. One day, young Horatio began to chew (咀爵) the chicle while he watched his father work. It did not 6 very good, but Horatio enjoyed chewing it. Then the young boy began to blow bubbles with the new chicle-which his father had made. Mr Adams had discovered bubble gum 7 . Mr Adams gave up trying to find a way to make rubber. 8 , he wanted to try to sell the new gum that he had made. He thought 9 people might like the taste, too.He was 10 . Soon, the new gum became more and more popular. | |||
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