下列命题中正确的是（ ）
A．空间三点可以确定一个平面
B．三角形一定是平面图形
C．若A、B、C、D既在平面α内，又在平面β内，则平面α和平面β重合
D．四条边都相等的四边形是平面图形
答案：B
题目分析：不在同一直线的三点确定一个平面，故A错，B对；共线的四点可以构成无数个平面，故C错；
正四面体的四个边都相等，但它不是平面图形，故D错.
故选B.
| 用方框中单词的正确形式完成短文。其中有两个词是多余的。 | |
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| Dear Han Yu, How are you? Thank you for your last letter! Now let me tell you about my favorite music and singer. In our class, everyone likes music very much. When we feel l out, we always listen to music to relax 2 . We all prefer pop music to classic music. But we like different __3 _ of music. Most students like music that's loud. But I have to be 4 and say I like music that is quiet and gentle. Many students like Zhou Jielun best. They think he is cool and he can write his original 5 .But Li Yuchun is my favorite pop singer. Because she has lots of __6_ and she is very beautiful. I like listening to her songs. I think her sound is sweet. I put up many of her 7 in my bedroom. She is the best and greatest singer in my heart. Next Saturday evening, she will have a concert in Wuhan. I will go to listen to her songs with my family and I _ 8_ to take photos with her. I think her songs must 9 _ you as much. Whatever you do, please don't 10 her concert. Let's meet at her concert ! Yours , Lin Lin |