阅读《当灾难突然降临的时候》，回答 后面题目。
        ①这是一个繁华的大都市，每天忙碌的人熙熙攘攘。八点三十分。装满乘客的地铁缓缓起动，向下一个站点驶去。
        ②当列车行驶到两个站点中间地段时，突然一片漆黑，车也慢慢停下来。车厢里的人一下像炸了锅似的，乱哄哄的如同马蜂窝从树上落到地下。有人大声骂司机：“真是个笨蛋!”也有人说：“养你们这些人有什么用，简直是一群废物，”
        ③司机解释道：“突然停电，这是一次意外事故，我马上与调度中心联系，”司机在黑暗中摸出手机，拔完号码后没有任何信号。
        ④这是一次意外停电，波及全市。始料不及的人们被这突如其来的停电搞蒙了，通讯中断，交通阻塞，整座城市近乎瘫痪。地铁里的人如同无头苍蝇，纷纷掏出手机与外界联系，却都没有信号。有人用打火机照亮，得到片刻的安慰。女人哄着不断哇哇哭叫的孩子。有人说：“我好不容易找到这份工作，迟到会被炒鱿鱼的。”有人说：“我与客商约好，签一份额为三百万元的合同，失约可能会使合同泡汤的。”也有人说：“等出去了，一定要起诉地铁公司。”黑暗像一只无情的大手，捏得人喘不过气来。哭声、骂声、叹息声不绝于耳。有人说，“等一等吧，地上的人不会不管我们的。”
        ⑤突然，黑暗中传出一个女孩稚嫩的声音：“妈妈，太阳落山了!”妈妈说：“宝贝，乖，天黑了，宝宝睡觉觉……”妈妈轻轻地哼起了摇篮曲。大家静静地听着，情绪渐渐地稳定下来，各自想着心事。此时，车厢里。
        ⑥不知过了多长时间，一个恐怖的声音打破了可贵的安静：“我心脏病犯了，没带药，谁有急救药……”旁边的人就问：“谁有药?”人们开始纷纷问着，一个接一个往下传。终于有人说：“我有。”接下来就听到急急忙忙的翻找声。
        ⑦“找到了!找到了!”那人边说边把小药瓶递给挨着他的人，这人接过药瓶又传。递给下一个人。没有人组织，药瓶却像接力棒那样被有序地传递着。
       ⑧“唉呀!”一声惊叫，药瓶不知传到谁的手里时不慎掉到地上，那人急忙俯下身在地上摸，整个车厢的人都俯下身在地上摸，仿佛药瓶会在黑暗中流窜到车厢的任何一个角落。终于有人说找到了，把药瓶又传下去。有过落地的教训，接药瓶的手就格外认真。药瓶很快就传到病人手里。黑暗中，不知是谁递过一瓶水，病人吃完药安静了下来。
       ⑨不知又过去了多长时间，与外界还是没有一丝联络，有人提议往下一出站口走，立即有人响应，有人用打火机把自己的衣服点着了照明。立即有人大声喊：“快灭掉，烟能使人窒息!万一引起火灾一个人也别想跑掉!”几个人手忙脚乱地把火扑灭了。刹那间，光明消失了，重新又回到黑暗的深渊。人们从来没像今天这样憎恨黑暗，也没有过像今天这样渴望光明。
       ⑩这时，有一个人大声喊道：“大家手牵手跟我走!”
       ⑪话音刚落，人们就主动地手拉着手排成了长队，那人又问：“有没有没牵手的?”当确认都牵好手后，那人就带头向下一出站口走去。黑暗中，那带头的人不时被绊倒，但他爬起来再走，不知摔了多少跤。后边的人有条不紊地跟着，没人说话，只有脚步声。
       ⑫人们终于从黑暗的地铁中走出来了，领头的人衣服刮破了，脸和手都碰出了血。更让人意外的是，人们发现，领头的这个人竟然是个盲人。

    Chinese students always complain that they're under great pressure and don't get enough sleep. But they may be surprised to learn that they're much luckier than their peers (同龄人) in South Korea and Japan, according to a recently published report, conducted by Shanghai Academy of Social Sciences on the physical and mental health of senior high school students in the three Asian countries.

    According to the study among nearly 6,000 high school students in the three countries, Chinese students are the tallest but poorest in physical fitness. Chinese students are on average 168.17cm, compared with 167.61cm for South Korean students and 164.70cm for Japanese students. Chinese students asked for sick leave more than students in other countries. About 72 percent of Korean students never asked for sick and injury leave during high school, compared to 53 percent for Japanese students and 45 percent for Chinese students. Moreover, Chinese students drink alcohol and smoke earlier than Korean and Japanese students. Around 70 percent of Chinese students have drunk alcohol, compared to 50 percent for Korean and Japanese students.

    According to the survey, Japanese and South Korean students suffer more pressure than their Chinese peers. Statistics (统计) show 16 percent of students in China said they experienced mental pressure often over the past year, while the numbers in Japan and South Korea were 33 percent and 47 percent respectively. Their worries come from “study”, “future after graduation”, “friendships” and “relationship with their parents”. To go with stress, 57.9 percent of Korean students sleep the least, less than six hours a day, compared with 46.6 percent of Japanese students and 30 percent of Chinese students.

    More Chinese students were satisfied with their physical appearance, double that of students in the survey from Japan and South Korea. The report said that over half of students in South Korea had tried to lose weight in the past year, while in China the figure was only 22 percent.

    In measures of self-worth, such as “I think I am valuable” and “I think I am successful”, “I can complete many things”, “I won't give up to failure,” Chinese students showed greater self-confidence than students in Japan and South Korea.

某物体做匀变速直线运动，其位移与时间的关系为x=0.5t+t²（m），则当物体的速度为3m/s时，物体已运动的时间为（   ）