布满水垢的水壶烧水，必然要消耗更多的燃料。清理水垢应在水壶中放些白醋或者是（）。A．

氧元素在自然界存在存在非常广泛，并不断循环转化．请回答下列问题：

（1）在地壳中存在氧元素的物质有二氧化硅、碳酸钙等，而在空气中存在氧元素的物质有______（至少写出2种）等．在人体内存在的化合物中，氧元素的质量分数最高的是______．

（2）以下为自然界中发生的一些变化：

①金属锈蚀；②化石燃料燃烧；③陆地、水生植物的光合作用；④人与动物的呼吸．从①--④中选择序号填空，其中消耗氧气的有______，产生氧气的有______．

（3）下列说法中正确的是______（填字母标号）．

A任何情况下呼吸纯氧均有益于人体健康；

B氧元素在自然界中既有游离态，又有化合态；

C含有氧元素的物质在化学反应中均是氧化剂；

D氧气有百利而无一害．

硫酸亚铁晶体（FeSO₄·7H₂O）在医药上作补血剂。某课外小组的同学欲测定该补血剂中铁元素的含量。实验步骤如下：

请回答下列问题：

（1）证明步骤①滤液中含有Fe²⁺的方法是：取样，先滴加KSCN溶液，再滴加_______，该过程的现象为：__________________________________________。

（2）步骤②加入过量H₂O₂的目的是：____________________________________。

（3）步骤③中反应的离子方程式是：_____________________________________。

（4）步骤④中一系列处理的操作步骤：过滤、_______、灼烧、_______、称量。

（5）若实验中铁无损耗，则每片补血剂中含铁元素的质量为_______g。

境内第一类医疗器械由什么机构审查（）

A.县级（食品）药品监督管理机构

B.市级（食品）药品监督管理机构

C.省、自治区、直辖市（食品）药品监督管理机构

D.国家食品药品监督管理局

E.卫生部

The richest man in America stepped to the podium and declared war on the nation’s school systems. High schools had become "obsolete" and were "limiting—even ruining—the lives of millions of Americans every year. " The situation had become "almost shameful. " Bill Gates, prep-school grad and college dropout, had come before the National Governors Association seeking converts to his plan to do something about it—a plan he would back with $ 2 billion of his own cash.

Gates’s speech, in February 2005, was a signature moment in what has become a decade-long campaign to improve test scores and graduation rates, waged by a loose alliance of wealthy CEOs who arrived with no particular background in education policy—a fact that has led critics to dismiss them as "the billionaire boys’ club. " Their bets on poor urban schools have been as big as their egos and their bank accounts.

Has this big money made the big impact that they—as well as teachers, administrators, parents, and students—hoped for The results, though mixed, are dispiriting proof that money alone can’t repair the desperate state of urban education. For all the millions spent on reforms, nine of the 10 school districts studied substantially trailed their state’s proficiency and graduation rates—often by 10 points or more. That’s not to say that the urban districts didn’t make gains.

The good news is many did improve and at a rate faster than their states’ 60 percent of the time—proof that the billionaires made some solid bets. But those spikes up weren’t enough to erase the deep gulf between poor, inner-city schools, where the big givers focused, and their suburban and rural counterparts. "A lot of things we do don’t work out," admitted Broad, a product of Detroit public schools and Michigan State who made a fortune in home building and financial services: "But we can take the criticism. "

The confidence that marked Gates’s landmark speech to the governors’ association in 2005 has given way to humility. The billionaires have not retreated. But they have retooled, and learned a valuable lesson about their limitations. "It’s so hard in this country to spread good practice. When we started funding, we hoped it would spread more readily," acknowledges Vicki Phillips, the director of K-12 education at the Bill ＆ Melinda Gates Foundation. "What we learned is that the only things that spread well in school are kids’ viruses. "

The business titans entered the education arena convinced that America’s schools would benefit greatly from the tools of the boardroom. They sought to boost incentives for improving performance, deploy new technologies, and back innovators willing to shatter old orthodoxies. They pressed to close schools that were failing, and sought to launch new, smaller ones. They sent principals to boot camp. Battling the long-term worry that the best and brightest passed up the classroom for more lucrative professions, they opened their checkbooks to boost teacher pay. It was an impressive amount of industry. And in some places, it has worked out—but with unanticipated complications.

Bill Gates believes that the high school systems（）

A. have failed to develop proper education programs for students

B. are running well except that they need enormous investments

C. have not made students academically ready for college education

D. have converted brilliant young students into dull learners

为何要对带式焙烧机炉罩内各段压力进行合理控制？