检验某未知溶液中是否含有Cl^﹣的方法是：取少量某未知溶液于试管中，先滴入几滴稀HNO₃，然后加入什么试剂（　　）

    Today is Tom's birthday. After school he walks past the shop on the street corner. He looks into the window. That pair of trainers(运动鞋) is still there. He really wants to have them.

    He is very sad and walks away. He knows his mother can't buy him anything, because his mother has no money. The trainers are too expensive. He doesn't go home at once , because his mother can notice（注意）his worry. So he goes to the park and sits on the grass. Then he sees a boy in a wheelchair(轮椅). He finds that the boy moves(移动) the wheels with his hands. Tom looks at him carefully. The boy has no feet. He looks at his own feet. "It's much better（更好）to be without shoes than (比) without feet." he thinks. There is no reason for him to feel so sorry and sad. He smiles and thinks he is happy.

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曲辕犁反映了华民族的创造力，不仅有着精巧的设计，精湛的技术，还蕴含着一些美学规律，其历史意义、社会意义影响深远。在当代农具设计中，曲辕犁仍有着很好的借鉴意义。曲辕犁制成于（　　）