题目

Everybody knows how to learn.Learning is  a natural thing.It begins the__41__we are  born.Our first  teachers are our families.At  home  we  learn  to talk and to__42__and feed ourselves.We learn these and other skills by__43__. Then we go to school.A teacher tells us_44__to learn and how to learn.Many teachers teach us，and we pass many tests and exams.Then people say we are__45__. Are we really educated? Let's__46__the real meaning of learning.Knowing facts doesn't __47__being  able to solve problems.Solving  problems requires creativity，not just a good__48__.Some people某不纯的碳酸钠固体样品，含有的杂质可能是硫酸钠、碳酸钙、硫酸铜、氯化钠、氯化钙中的一种或几种．为确定其杂质的成分，进行如下实验：①取该样品溶于水，得无色溶液．②向适量上述溶液中，滴加过量的氯化钡溶液，出现白色沉淀，过滤．③向步骤②所得沉淀中加入过量的稀硝酸，沉淀部分消失并产生气泡．④取步骤②的滤液，加入稀硝酸、硝酸银溶液，出现白色沉淀．下列分析中，正确的是（　　） A、不能确定杂质是否含有氯化钠和氯化钙B、杂质一定只含有硫酸钠C、若步骤②改为滴加硝酸钡溶液则通过步骤④可以确定杂质是否含有氯化钠D、若步骤②改为滴加氯化钡溶液至恰好完全反应则可以肯定杂质不含氯化钠