下列说法错误的是（ ）
A．二次函数y=-2x2中，当x=0时，y有最大值是0
B．二次函数y=4x2中，当x＞0时，y随x的增大而增大
C．在三条抛物线y=2x2，y=-0.5x2，y=-x2中，y=2x2的图象开口最大，y=-x2的图象开口最小
D．不论a是正数还是负数，抛物线y=ax2（a≠0）的顶点一定是坐标原点
A、a=-2＜0，抛物线开口向下，当x=0时，y有最大值是0，正确；
B、二次函数y=4x2中，当x＞0时，y随x的增大而增大，正确；
C、因为|2|＞|-1|＞|-0.5|，所以，y=2x2的图象开口最小，y=-0.5x2的图象开口最大，错误；
D、不论a是正数还是负数，抛物线y=ax2（a≠0）的顶点一定是坐标原点，正确．
故选C．
| 阅读理解 |
| Dear Wei Lin: It was really good to hear from you again, but I was surprised to learn that you started smoking soon after you went to the U.S. You said that smoking helps you relax. This might be true, but there are better ways to relax . And doctors tell us that smoking can only help us relax for a short time. I remember when you were in Taiwan, you were afraid that the air pollution might harm (对...有害处) your health. But don't you think you're also harming your health when you smoke? Smoking can hurt other people's health, too. And I've heard that some people have gotten cancer from second-hand smoke. So for your own health and for the health of others, please give up smoking … Yours, Alex |
| 1. From the letter, we know that Wei Lin _______. |
[ ] |
| A. couldn't smoke B. started smoking when he was old C. started smoking when he was in Taiwan D. learned to smoke after he went to U.S. |
| 2. Why did he smoke? |
[ ] |
| A. He was too busy. B. He enjoyed smoking. C. He needed it. D. He wanted to relax. |
| 3. What does the writer think of smoking? |
[ ] |
| A. He likes it very much. B. It is good to relax. C. It is harmful to health. D. It is OK to smoke a little. |
4. Which of the following is NOT true according to the passage? |
[ ] |
| A. Alex asked Wei Lin to give up smoking. B. Alex was Wei Lin's friend. C. Alex hated smoking because it hurt health. D. Wei Lin got cancer. |
| 5. According to the writer, smoking is _______. |
[ ] |
| A. harmful to smokers' health B. not good to others' health C. as harmful as air pollution D. All of the above |