气象专家和医学专家认为，灰霾天对人体健康的危害比沙尘暴还大．世界公认PM2.5是造成灰霾天的主要因素．PM2.5可直接进入肺，它随着血液循环首次经过如图中心脏各腔的先后顺序是（  ）

    Volunteering to help people in need combined____travelling to faraway places is ____ new trend in the travel industry. It is called voluntourism. People travel to other countries, learn languages and other cultures and gain new____ ( experience ). Recent statistics show that in the past few years voluntourism ____ ( be) one of the fastest - growing areas of tourism, More than 1.6 million people around the world are volunteers in other countries. They work in orphanages (孤儿院), help build schools, assist in hospitals and do farming work in____ (develop) countries. There are many reasons ____ people want to engage in voluntourism, Students see it as a gap year after school, while others ____ (simple) want to take time out from a job and do something else. However, some voluntourists do not see volunteering ____ what it is. They only think it is a cheap way ____（travel）and don't really want to get____ （involve） in hard work.

圣诞夜，一场好球胜过一顿大餐。北京时间12月26日，NBA圣涎大战再次进入球迷视线。“圣诞”原是纪念谁的涎辰?(    )

①CO₂+Na₂CO₃+H₂O=2NaHCO₃，CO₂通入饱和碳酸钠溶液会产生白色沉淀，CO₂不溶于饱和碳酸氢钠溶液。

②碳酸钠、碳酸氢钠在不同温度下的溶解度如表：

【进行实验1】验证CO₂通入饱和碳酸钠溶液会产生白色沉淀

①配制碳酸钠溶液：取100 g 20 ℃的水，加入35 g碳酸钠固体，搅拌后，静置，取上层清液。

②利用装置一进行实验，连续5分钟向新制得的碳酸钠溶液中通入CO₂。

【发现问题】实验中，始终没有看到沉淀现象，原因是什么呢？雯雯进行了以下实验探究。

【进行实验2】