I was only nine years old when I learnt how to use a computer．My mother is my first teacher．I know how to type，how to copy a file，and how to visit a website on the Internet．One day，mom was not at home．I turned on the computer and began to learn how to chat on net．The first one I met there was a boy called Tom．He greeted me politely．When he knew that I was only a 9-year-old girl and almost knew nothing about chatting on net，he started showing me how to use the chatting tools，how to download，and how to send e-mails．I learnt a lot from him．

①认真履行接受规定年限的义务教育的义务，不得中途辍学

②认真履行开展自主学习、合作学习、探究学习，养成良好的学习习惯、提高学习能力的义务

③认真履行按时入学的义务④认真履行遵守法律和学校纪律，尊敬师长，努力完成规定学习任务的义务

实验小组同学发现，相同情况下烧开一壶水，用天然气比用煤气的量要少，其原因是由于这两种燃料具有不同的（　　）

下列物质不能用于鉴别氢氧化钠溶液和稀盐酸的是（　　）