按要求改写句子。

1. Your aunt left for Changchun two days ago. (改为反意疑问句) 

    Your aunt left for Changchun two days ago, _____ _____? 

2. Lucy is 10 years old. Mary is 14 years old. (合为一句)

   Mary is _____ years _____ than Lucy.

3. I think Jason's is the best. (改为否定句)

    I _____ think Jason's _____ the best.

4. Bill is taller than any other student in our class. (改为同义句)

    Bill is _____ _____ student in our class.

5. The weather in Sanya is still warm in winter. (对画线部分提问)

    _____ the weather _____ in Sanya in winter?

圆管层流，实测管轴线上流速为4m/s，则断面平均流速为（）。

A．4m/s

B．3.2m/s

C．2m/s

D．1m/s