王老师在某工厂废水处理池中提取一瓶含有AgNO₃、Cu(NO₃)₂和Al(NO₃)₃的工业废水样品带回实验室。静置，取上层清液，加入锌粉，充分反应后过滤，得到滤液甲和滤渣乙。请你和你的化学小组完成以下探究（不考虑杂质干扰）。

【提出问题】滤渣乙是什么？

【做出猜想】小明认为滤渣乙是银，小东认为滤渣乙是银和铜，小华认为滤渣乙是银、铜和铝，你认为滤渣乙还可能是银、铜和锌。

农业区位选择的实质是（　　）

阅读理解

    Being a fit person is very important for lots of people today. In fact, some people like running and exercising so much that they become obsessed with it.
Thanks to fitness apps and gadgets (小玩意), people can measure every step they take, what they eat and every calorie they burn. They might wear a special watch on their wrist, shoes on their feet or use different apps on their phone to measure their movements. Importantly, once they have done that they post it on social media. All of their friends can see where they've run, what workout (健身) they've done or what they had for lunch.
    Doctor Daniel Kraft from the United States makes gadgets for people like this. He told the Digital Health Summit that these people would become the norm (常态) in the future.
    I think he's right. I can see the world of health and fitness changing around me every day.
    One of my friends, named Anna Palmer, is always using the latest fitness gadgets. Her favorite is an app called Argus. She uses it on her phone to measure her steps, heart rate and amount of calories she has burnt. She always posts these things and pictures of herself exercising on social media. “It's nice to know exactly how much exercise you have done rater than guess,” she said. “That way I can keep pushing myself to improve.”
    Also, my friends in China use Wechat to compare how many steps they've taken each day. They are very competitive about this!

应用元素周期律，判断下列语句，其中正确的组合是
①碱金属单质的熔点随原子序数的增大而降低②砹（At）在第VIIA族，其氢化物的稳定大于HC1 ③硒（Se）在第VIA族，SeO₂的氧化性强于SO₂ ④第二周期非金属元素的气态氢化物溶于水后，水溶液均为酸性⑤铊（Tl）与铝同主族，其单质既能与盐酸反应，又能与氢氧化钠溶液反应⑥第三周期金属元素的最高价氧化物对应水化物，其碱性随原子序数的增大而减弱

一个样本，各个数据的和为515，如果这个样本的平均数为5，那么这个样本的容量是____。