阅读下列短文，从每题所给的四个选项中（A、B、C和D），选出最佳选项，并在答题卡上将该项涂黑。

    Teachers at American colleges and universities have many different teaching methods. Some teachers give homework and check the work every day, and students in their classes have to take many exams. Some teachers give only writing tasks. Some teachers always follow a course outline and usually use the textbook, while others send students to the library for tasks.

    The atmosphere in some classrooms is very formal. Students call their teachers Professor Smith, Mrs. Jones and so on. Other classrooms have an informal atmosphere. Students and teachers discuss their ideas together. Teachers dress informally, and students call them by their first names. American teachers are different in their teaching styles.

At most American colleges and universities, libraries and learning centers can be used by students at any time. They can often use tape recorders, video machines and computers. They can buy books, notebooks and other things at campus stores. There are also services to students. They can get advice on their problems from their teachers. Colleges and universities usually offer facilities to students. Some schools have swimming pools and tennis courts. Most have fast food restaurants.

请回答：

依次填入下面一段文字横线处的语句，衔接最恰当的一组是（         ）

甲型H1N1流感为急性呼吸道传染病，             有时会出现呕吐和腹泻，严重的会导致死亡。
①人群对甲型H1N1流感病毒普遍易感，并可以人传染人，
②与以往或目前的季节性流感病毒不同，该病毒毒株包含有猪流感、禽流感和人流感三种流感病毒的基因片段，
③包括发热、咳嗽、头痛、肌肉和关节痛、咽痛以及流涕等，
④表现出的症状和感染和其他流感类似，
⑤其病原体是一种新型的甲型H1N1流感病毒，以往从未在人群中传播过，

某同学做“探究弹力和弹簧伸长的关系”的实验，他先把弹簧平放在光滑水平面上使其处于自然状态，用直尺测出弹簧的原长L₀，再把弹簧竖直悬挂起来，挂上钩码后测出弹簧伸长后的长度L，把（L﹣L₀）作为弹簧的伸长量x．这样操作，由于弹簧自身重力的影响，最后画出的图线可能是下列所示选项图中的哪一个？该图象的斜率与弹簧的劲度系数的关系如何？       

①幸而府上是向来宽洪大量，不肯和小人计较的。              ②阿呀阿呀，我真上当。

③她来求我荐地方，我那里料得到是瞒着她的婆婆的呢。        ④这回我一定荐一个好的来折罪……。

⑤总是我老发昏不小心，对不起主顾。                         ⑥我这回，就是为此特地来说说清楚的。

⑦对不起，四老爷，四太太。