— Mimi, what does your sister do?

— She is a nurse.

— And your mother?

画线部分的意思是          

(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.

若函数$f\left(x\right)=x^2+ax+b$有两个零点$\cos \alpha ,\cos \beta$，其中$\alpha ,\beta \in \left(0,\mathrm{\pi }\right)$，那么在$f\left(-1\right),f\left(1\right)$两个函数值中  (    )