在二战的危机关头，世界反法西斯国家建立了统一战线。其标志是（   ）

下列推论正确的是（    ）

设，满足约束条件 则的最大值为（     ）

根据短文内容, 回答下列问题。
    Lin Hai is thirteen. He's a student in a school in Tianjin. He gets an e-mail from Tom White. Tom is a student in the USA. Here is the e-mail.
Dear Lin Hai,  
    My name is Tom White. I am from New York, USA. I am fourteen years old. I'm a schoolboy. I want to make some friends in China.
    There are eleven boys and nine girls in my class. We come from different countries—Japan, the USA, the UK and Canada. But at school we all speak English. My parents are from France (法国). They speak French and English, so I can speak English and French well. But I want to learn Chinese. Can you help me? There are many Chinese people in New York and I want to talk with them.
I can play chess well, and I can play many sports. How about you?
Please write to me soon.
Yours,  
Tom

类推是科学学习中常用的思维方法。以下说法错．误．的是（   ）

①铁能置换出硫酸铜溶液中的铜，所以也能置换出硫酸钠溶液中的钠

②NaOH 能使无色酚酞溶液变红，所以 Cu(OH)₂ 也能使无色酚酞变红

③NH₄Cl 不能与碱混合使用，所以 KNO₃ 也不能与碱混合使用

④碳酸盐能与盐酸反应生成气体，所以能与盐酸反应生成气体的一定是碳酸盐