Elias was a poor black worker in south Africa. The time ____he first met Nelson Mandel was a period of his life .he was twelve years old at that time. Nelson Mandel was a black lawyer to____ he turned for advice .Nelson offered____(guide) to poor black people on their____(illegal) problems. Elias needed Mandel's help____he had little education. The school ____he studied for only two years was three kilometers away .he had to drop out of school because his family couldn't pay the school fees____the bus fare .He couldn't read or write well .After trying hard, he got a job in a gold mine ,____ he didn't have passbook to live in Johannesbug and he worried about ____he would become out____work.

2015年3月22日“是第22届“世界水日”，主题是“水与能源”．再次提醒人类关注水资源问题，珍惜水，节约水，保护水资源．据媒体报道，某市区域内出现了两条“黄河”，经过各级环保部门调查发现，“黄河”形成的原因是这两处的上游均有较多的标准件生产企业，这些企业在生产过程中，都要对钢铁半成品进行酸洗除锈处理，然后对过量的盐酸用氢氧化钙中和后流入河道．请用学过的科学知识回答下列问题：

（1）河水成“黄色”的原因是因为河水中含有大量　____ 所致．（填写离子符号）

（2）当地农民用此河水浇灌农作物，大多农作物竟然出现了萎蔫和枯死的现象，专家分析称是由于河水中含盐量过高所致，请说出植物萎蔫和枯死的原因是____ ．

（4）河水成“黄色”后水的物理性状发生了改变，取等体积的蒸馏水和被污染的“黄水”称量后“黄水”的质量大于蒸馏水的质量，说明被污染的水的____ 发生了改变．

一张课桌的面积大约是30（　　）