问题
单项选择题
以下结论中正确的是（）。
A．若方阵A的行列式|A|=0，则A=O
B．若A2=O，则A=O
C．若A为对称阵，则A2也是对称阵
D．对于任意n阶方阵A和B，有(A+B)(A-B)=A2-B2
答案
参考答案：C
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以下结论中正确的是（）。
A．若方阵A的行列式|A|=0，则A=O
B．若A2=O，则A=O
C．若A为对称阵，则A2也是对称阵
D．对于任意n阶方阵A和B，有(A+B)(A-B)=A2-B2
参考答案：C
My wife Julie and I were out on the road that______around where we live, when we saw a（n）______ worn-out dog stumbling (蹒跚) painfully up the road. We stopped, bent down, talked __38____to the dog and patted it. I______ and there was a collar with a phone number. I called ______ no one answered. The dog was painfully thin. So Julie ran home to get some of our dog’s ______ while I tried to encourage the dog. After Julie came back, we sat down on the sidewalk ______ our new friend made short work of the food. ______we got her home. After trying for many times we got a ______from the number. A lady came around with a bunch of flowers for us. She ______ that Tara had been her father’s dog. She was very old and got lost that morning. So, Tara was ______returned home. Here is the _____of the story: Actually Julie and I were out that morning because I was leaving. She was trying to______me to come back, but I wasn’t hearing ______ that made that sound likely. I was ______ to turn and go when an old black dog walked between us and almost fell down. Suddenly we had something more______than our problem to worry about. There was a creature ______ right before us and we had to work together to help it. We did help it. And here I am writing the story in my own home, in my own family. In the______ “Love Is Not a Fight” Warren Barfield talks about marriage. At one point he sings, “And if we try to leave, ______ God send angels (天使) to guard the door.” Sometimes angels come ______ as dogs.
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