下图反映的美国失业率达到最高峰的主要原因是(     )

下列属于最简二次根式的是（　　）

阅读理解:阅读下列短文，然后从每题所给的四个选项(A、B、 C、和D)中，选出最佳选项。

    Mr. and Mrs. Green were going on their vacation with their children, and they had to arrive at the airport before 11:40 the next morning. “It takes us thirty minutes to get there in a car,”Mr. Green said, “so we must get ready before eleven o'clock. Don't be late.”

At ten fifty, all of them were busy doing something, but Mrs. Green wasn't. She sat quietly in a chair in the garden and enjoyed the good weather.

Mr. Green and the children were surprised, but Mrs. Green said, “Don't worry! Before I went to bed last night. I made all our clocks and watches twenty minutes ahead(提前). Now we can get to the airport early.”

如图是太阳系局部图，黑点表示小行星．回答下列下题．