____

血友病是一种X染色体上隐性基因控制的遗传病．现有一对表现型均正常的夫妇，妻子是致病基因的携带者．那么该对夫妇生一个患血友病的女儿的概率为（　　）

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.”