已知盐水若干克，第一次加入一定量的水后，盐水浓度为3%；第二次加入同样多的水后，浓度变为2%，那么第三次加入同样多的水后盐水的浓度为（）。
A．1.6%
B．1.5%
C．1.2%
D．1%
参考答案：B
解析：
设原盐水的重量为x克，每次加入水y克。因为溶质数量始终不变，可得到： (x+y)×3%=(x+2y)×2%，解得x=y。 那么第三次加入同样多的水后盐水的浓度为： (x+y)×3%÷(x+3y)=6%x÷4x=1.5%，故本题正确答案为B。
| 从方框中选出最佳答案完成对话，有两项是多余的。(填序号即可) | |
| A: Hi, Li Fang. You look unhappy, what's wrong? B: 1__________ A: I don't think that's a good idea. We aren't children any longer. B: That's right. 2__________ A: really? My father allows me to make friends with all of my classmates, but my mother doesn't agree with him. B: 3__________. My mother doesn't let me play computer games when I am free, either. A: I think she is right. We mustn't spend too much time playing computer games. 4__________. He thinks a girl should have long hair. B: That's funny. My father always tells me not to ride my bike to school. He thinks it's too dangerous. A: I don't think it is, but you should ride slowly. It's too dangerous to ride fast. B: You are right. 5__________ A: I think we need to talk to them and make them understand us. B: You are right.
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