根据二次根式有意义的条件，得a-1≥0，解得a≥1．
故a的取值范围为a≥1．
—Shall we go to the book show tomorrow?
—__________ [ ]
A. Thank you.
B. It's very nice of you.
C. That's a good idea.
D. It doesn't matter.
咨询时，不应该打断求助者的叙述，免得引起方的反感，哪怕方叙述的内容并不重要。
