热网载热质为蒸汽的汽网时，其供热距离不能太大，一般是（）km。
A.5～7
B.3～5
C.7～9
D.8～10
参考答案：A
根据对话内容，在每个空白处填入一个适当的单词，使对话完整，合乎情景。
A：Excuse me, can I (1)________your bike?
B：(2)__________ , I don't have one. Ask Yu Yan. I think he has (3)____________.
A：OK. Thanks a (4)________________. Yu Yan, do you (5)____________a bike?
C：Yes, I do. (6) _________it's only an old one.
A：That's OK.(7) ____________I borrow it, please?
C．Certainly. But please (8) _______it back soon.
A：OK. Thanks(9) _______much.
C：(10)________at all.
地球半径为R，地球表面的重力加速度为g，若高空中某处的重力加速度为g/2，则该处距地球表面的高度为
A．()R
B．R
C．
D．2R
)R