对于条件：①两条直角边对应相等；②斜边和一锐角对应相等；③斜边和一直角边对应相等；④直角边和一锐角对应相等；以上能断定两直角三角形全等的有（　 　）

如图，抛物线y=ax²+bx+c（a＞0）的对称轴是直线x=1，且经过点P（3，0），则a-b+c的值为 (       ）

社会上食品造假事件时有发生．小明的奶奶从自由市场上购买了一箱牛奶，小明想知道牛奶是否掺水．通过查阅资料得知，在牛奶中掺水后，掺水含量与牛奶密度的关系如下表所示．小明取100ml这种牛奶，测得它的质量为102.2g，则这种牛奶（    ）

看图，选择合适的介词填空。

- Can I help you, Tim?

- Yes. I need a ruler. Give me a ruler, please.

- Where is it?

- It's____ the bag. Give me a pencil case, please.

- Where is it?

- It's____ the table. Give me a ball, please.

-Where is it?

- It's____ the table.

- Here you are.

- Thank you. Oh, where is my chair? I can't find it.

- Don't worry. It's here,____ the table.