某病的流行曲线只有一个高峰，所有病例都集中在该病常见潜伏期这个时间段内，则可断定此暴

逐渐开始和终止，又易复发的是（）。

A.心房扑动

B.三度房室传导阻滞

C.尖端扭转室速

D.阵发性室上性心动过速

E.窦性心动过速

碘溶于碘化钾溶液中形成I₃^-，并建立如下平衡：I₃^-═I^-+I₂．实验室可以通过氧化还原滴定法测定平衡时I₃^-的浓度

实验原理：为了测定平衡时的c（I₃^-），可用过量的碘与碘化钾溶液一起摇动，达平衡后取上层清液用标准的Na₂S₂O₃滴定：2Na₂S₂O₃+I₂=2NaI+Na₂S₄O₆．由于溶液中存在I₃^-═I^-+I₂的平衡，所以用硫代硫酸钠溶液滴定，最终测得的是I₂和I₃^-的总浓度，设为c₁，c₁=c（I₂）+c（I₃^-）；c（I₂） 的浓度可用相同温度下，测过量碘与水平衡时溶液中碘的浓度代替，设为c₂，则c（I₂）=c₂，c（I₃^-）=c₁-c₂；

实验内容：

1．用一只干燥的100mL 碘量瓶和一只250mL碘量瓶，分别标上1、2号，用量筒取80mL 0.0100mol．L^-1KI于1号瓶，取200mL 蒸馏水于2号瓶，并分别加入0.5g过量的碘．

2．将两只碘量瓶塞好塞子，振荡30分钟，静置．

3．分别取上层清液20mL用浓度为c mol/L标准Na₂S₂O₃溶液进行滴定．1号瓶消耗V₁mL Na₂S₂O₃溶液，2号瓶消耗V₂mL Na₂S₂O₃溶液．

4．带入数据计算c（I₃^-）

试回答下列问题

（1）标准Na₂S₂O₃溶液应装入______（填“酸式滴定管”或“碱式滴定管”），原因是______．

（2）碘量瓶在震荡静置时要塞紧塞子，可能的原因是______

（3）滴定时向待测液中加入的指示剂是______，到达滴定终点时的现象为______．

（4）用c、V₁ 和V₂ 表示c（I₃^-）为______mol•L^-1．

（5）若在测定1号瓶时，取上层清液时不小心吸入了少量的I₂固体，则测定的c（I₃^-）______（填“偏大”、“偏小”或“不变”）

根据《合同法》，关于违约责任的说法，正确的是（）。

A.违约责任以无效合同为前提 

B.违约责任可由当事人在法定范围内约定 

C.违约责任以违反合同义务为要件 

D.违约责任必须以支付违约金的方式承担 

E.违约责任是一种民事赔偿责任

The Federal Government

联邦政府

The Constitution
When America broke away from Britain in 1775, she did not adopt a British Constitution. The British have always had an unwritten constitution, whereas every item of the American Constitution is clearly written down and numbered, and can only be changed by a two-thirds majority vote of Congress.
Yet in their different ways, the American and British forms of government did have one thing in common. They were both democratiC．As for American society, it was more democratic than British society, in that it paid less attention to class or wealth.
Political Parties
There are two major political parties in the USA, the Democratic Party and the Republican Party. The policies of the two parties are not basically opposed to one another. Inside both parties there moderates and right-wringers, though the Democrats still have the reputation of being somewhat more liberal than the Republicans. Voters are influenced by family traditions, but there are plenty of ordinary people who vote according to their hopes, fears and beliefs.
There are no other political parties that can compete with the two big parties, though there are occasionally independents who stand for the presidency and manage to collect quite a few votes. There are no left-wing parties. Most Americans are in favor of free enterprise, believing that it may one day help them fulfill the American Dream.
The President and Congress
The President of the USA has more power than any other president in the democratic world- except the French President. It is he who formulates foreign policy and prepares laws for the home front. He is leader of the nation and Commander-in-Chief of the Armed Forces. He represents the USA and, since the USA is a super power, the eyes of the whole world are on him. The fate of the world is in his hands, or so many people believe, and one careless, ill-prepared speech by him could precipitate a crises.
Actually, a great deal of the President’s power is controlled by Congress, the American name for "parliament". It is Congress that declares war, not the President. Unlike the Prime Minister of Great Britain, or of Germany, he can make a treaty with a foreign power. But this treaty must be debated and agreed by Congress before it comes into force. The same control applies to laws at home. Congress has (on several occasions) refused to ratify treaties or given approval to laws proposed by the President. The USA is the only country, apart from France, where a president can rule with a parliament, the majority of whose members do not belong to his own political party.
Some Americans have the feeling that idealism has gone out of politics and that personal ambition and money have taken its place. The election campaign fro the Presidency is unique in the amount of money poured into it. The wooing of voters lasts for months. But before the campaign for the election of the President can begin, each political party has to choose its candidate for the Presidency. This can lead to some very close contests. Men aspiring to be elected as the party candidate employ top public relations and advertising men, who invent clever catch phrases and set about "selling" their man. There are whistle stop tours by train, by plane, by car. The candidate delivers countless speeches and shakes countless hands. This razzamatazz typifies American enthusiasm and extravagance.
Big money is necessary to support a presidential candidate’s campaign and the candidate himself must be rich enough to pay his share. An attractive wife is an advantage, too. Money is also needed to become the Governor of a state, or a successful Senator, or member of the House of Representatives. Yet from this small group many excellent men have become President, and the same is true of members of Congress.
It is very unlikely that the President could ever become a dictator. Congress, the press and the people between them rule out such a possibility.
The Supreme Court
Perhaps the most effective safeguard of democracy is the Supreme Court, for one of its objects is to protect the individual against the government. It has the authority to cancel a law which it considers violates the Constitution. The Court sits for at least four days a week and any individual who has a grievance against the government can apply to it for help.
The Supreme Court goes way back to 1787, the days of the Founding Fathers, and is one of the cornerstones of American democracy. It gives judgement in disputes between States, or between a State and the Federal Government, and without invitation can declare a law made by Congress to be unconstitutional. The great Jefferson, who drew up the Declaration of Independence, saw to it, too, that there was a Bill of Rights which every American could thrust under the nose of anyone who tries to rob him of his freedom as a democratic citizen.

One thing in common between the American and British form of government is that they were both______.

A．constitutional


B．unconstitutional
C．democratic


D．interested in class and wealth

数据字典是软件需求分析阶段的最重要的工具之一，其最基本的功能是什么

A．数据库定义

B．数据通信

C．数据定义

D．数据维护