题干

家猫体色由X染色体上一对等位基因B、b控制,只含基因B的个体为黑猫,只含基因b的个体为黄猫,同时含B和b基因的个体为玳瑁猫,下列说法正确的是(    )

A:玳瑁猫互交的后代中有25%雄性黄猫

B:玳瑁猫与黄猫杂交后代中玳瑁猫占50%

C:为持续高效地繁育玳瑁猫,应逐代淘汰其他体色的猫

D:只有用黑猫和黄猫杂交,才能获得最大比例的玳瑁猫

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D

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    Once when I was facing a decision that involved high risk, I went to a friend. He looked at me for a moment, and then wrote a sentence containing the best advice I've ever had: Be bold and brave — and mighty forces will come to your aid.

    Those words made me see clearly that when I had fallen short in the past, it was seldom because I had tried and failed. It was usually because I had let fear of failure stop me from trying at all. On the other hand, whenever I had plunged into deep water, forced by courage or circumstance, I had always been able to swim until I got my feet on the ground again.

    Boldness means a decision to bite off more than you can eat. And there is nothing mysterious about the mighty forces. They are potential powers we possess: energy, skill, sound judgment, creative ideas — even physical strength greater than most of us realize.

    Admittedly, those mighty forces are spiritual ones. But they are more important than physical ones. A college classmate of mine, Tim, was an excellent football player, even though he weighed much less than the average player. “In one game I suddenly found myself confronting a huge player, who had nothing but me between him and our goal line,” said Tim. “I was so frightened that I closed my eyes and desperately threw myself at that guy like a bullet — and stopped him cold.”

    Boldness — a willingness to extend yourself to the extreme—is not one that can be acquired overnight. But it can be taught to children and developed in adults. Confidence builds up. Surely, there will be setbacks (挫折) and disappointments in life; boldness in itself is no guarantee of success. But the person who tries to do something and fails is a lot better off than the person who tries to do nothing and succeeds.

    So, always try to live a little bit beyond your abilities—and you'll find your abilities are greater than you ever dreamed.