将O₂、C、CO、CuO、Na₂CO₃、稀硫酸6种物质两两混合，一定条件下发生的化学反应共有（   ）

下列实数:0．020020002……中,无理数有(  )个．     

Rt△ABC中，∠ACB=90°，若∠ACD=50°，则与∠BCD相邻的外角度数是（　　）

有关中国自然区域内部差异及其对人类活动影响的叙述，正确的是(     )

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    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!