下列命题中是真命题的是（ ）
A．∀θ∈[0，π），∃α∈R使得直线ax+y+1=0的倾斜角为θ
B．曲线C：ax2+by2=c表示双曲线的充要条件是ab＜0
C．到两定点（-2，4），（4，-4）距离和为12的点的轨迹是椭圆
D．到两定点（-2，0），（2，0）距离差的绝对值为4的点的轨迹是双曲线
A：θ=90°时，直线的斜率不存在
B：ax2+by2=c可化简为
+x2 c a
=1，若曲线表示双曲线，则y2 c b
•c a
＜0⇒ab＜0且c≠0c b
C：根据椭圆的定义可知
=10＜12符合椭圆的定义(-2-4)2+(4+4)2
D：由于两定点（-2，0），（2，0）之间的距离为4，则到两定点（-2，0），（2，0）距离差的绝对值为4的点轨迹是射线，而不是双曲线
故选：C
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| There will be a lot of problems in a teenager's life. They are just a part of 1 to be an adult. However, 2 they are dealt with well, they won't become real problems. Some parents don't understand their teenage children very well. They don't 3 their wishes or different moods (情绪) at this special time of life. They don't understand that it is quite 4 for teenagers to become adults. That is also the reason that teenagers 5 don't know what to do and get angry easily. I think I'm a good example of that kind of person. I ever had a(n) 6 like that. When I just went to the middle school, I 7 a lot of problems for my teachers. I didn't 8 the class rules and kept talking to my classmates in class. 9 the head teacher called my parents. After they learned 10 had happened, my parents took me home. To my surprise, they weren't 11 with me. They talked with me about the problem and tried to find out the reason for the problem 12 they could solve it. 13 their help, the problem was solved in the end. And I was really 14 for what they did for me. When there's a problem in a teenage child's life, the parents should find a good opportunity to have a(n) 15 with the child first. Then they should try to solve the problem together. This is good for both of the parents and the child. | ||||
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