使材料丧失正常工作能力的应力称为（）。A、危险应力 B、工作应力 C、安全应力 D、

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Dear Mary,

    Thanks for your letter and your nice photos. You really have a big family.

From the photos, I also know you have a pet dog. It's great.

    I like animals very much, too. My favorite animals are pandas. They're so

cute and they're from China. Do you know the two pandas "Lele"and

"Yingying"? They're in Hong Kong now. I think people all like them. Do you

know when the World Animal Day is? It's October 4th.

    I like animals but my parents don't. They think they're dirty. I want to have

a pet dog very much. Dogs are people's good friends. They're clever. But in

China, it's not easy to have a pet dog. We must pay much money on it. So

please give me more photos about your dog, OK?