兔子的毛色是由一组复等位基因控制的．C⁺控制野鼠色，对其他3个复等位基因为显性；C^ch控制灰色，对C^h和C^a为显性；C^h控制喜马拉雅白化，对C^a为显性；C^a是一种突变型，不能形成色素，纯合时兔子毛色为白色．以下分析正确的是（　　）

(C)

    John was a very strong man. Once he got a job of cutting wood in a forest. His boss gave him an axe(斧子) and showed him the area where he should work.

    The first day, John cut down 18 trees. "Good," the boss said, "keep it up!" John was encouraged by the words of the boss and tried harder the next day, but he could only cut down 15 trees. The third day he tried even harder, but he could only cut down 10 trees. Day after day he was cutting down fewer and fewer trees.

    "I must be losing my strength," John thought. He went to the boss and said that he could not understand what was going on. "When was the last time you had sharpened(使锋利) your axe?" the boss asked. "Sharpened? I had no time to do it. I have been very busy trying to cut more trees." said John.

    The moral of this story: While John was working hard, he didn't think about how to do his job better.