有一渡船正在渡河，河宽200m，船在静水中的速度为4m/s，水流速度为3m/s，则船最短的渡河时间为____ s，船最短的渡河距离为____m．

若两个非零向量$\overrightarrow{a}$、$\overrightarrow{b}$满足|$\overrightarrow{a}$+$\overrightarrow{b}$|=|$\overrightarrow{a}$﹣$\overrightarrow{b}$|=$\frac{2\sqrt{3}}{3}$|$\overrightarrow{a}$|，则向量$\overrightarrow{a}$+$\overrightarrow{b}$与$\overrightarrow{a}$﹣$\overrightarrow{b}$的夹角为（　　）

完型填空。

    We know health is very important 1 us. So we must keep2. How to do it? We should do more exercises and eat healthy food. We can play sports. For example, we can play football and basketball. We can run. We can attend the high jump and long jump. If you don't  like 3sports. You can go for 4walk every day. It's very good .We should eat fruit, vegetables and other healthy food. We should eat less sweet food and fried food. Breakfast5important. We must eat breakfast every day.

“沧海桑田”这个成语，从地理学的角度可以理解为   （  ）