自然资源的基本特征有哪些？

S_n为等差数列{a_n}的前n项和，S₅＞S₆，S₆=S₇，S₇＜S₈，以下给出了四个式子：

①公差d＜0；

②a₇=0；

③S₉＞S₄； 

④S_n的最小值有两个．

其中正确的式子共有（　　）

A．1个

B．2个

C．3个

D．4个

短文改错（共10小题；毎小题1分，满分10分）

下面短文有10处语言错误，请在有错误的地方增加、删除或修改某个单词。

增加：在缺词处加一个漏字符号（＾）.并在其下面写出该加的词。

删除：把多余的词用斜线(\)划掉，

修改：在错的词下划一横线，在该行右边横线上写出改正后的词。

注意：1.每处错误及其修改均仅限一词.

2.只允许修改10处，多者（从第11处起）不计分

In 1929, three years after his flight over the North Pole, the American explorer, R.E. Byrd, successful flew over the South Pole for the first time. Therefore, at first, Byrd and his men were able to take great many photographs of the mountains that laid below, they soon ran into serious troubles. At one point, it seemed sure that their plane would crash. It could only get over the mountains if it raised to 10,000 feet. Byrd at once ordered his men to throw away two heavy food sacks. The plane was then able to rise and it cleared the mountains by 400 feet. Byrd then knew that he would be able to reach the South Pole which was 300 miles away, for there was no more mountains at sight. The aircraft was able to fly over the endless white plains without difficult.

（2010）一般来说，在握手的时候，先伸手者为（）。

A.长辈

B.晚辈

C.上级

D.下级

E.女士

对医德修养的意义的正确表述是

A．使医务人员养成良好的医德品质和人格，使医疗卫生单位形成良好形象，在医疗竞争中立于不败之地
B．使医务人员养成良好的医德品质和人格，使医疗卫生单位形成良好的医德医风，从而促进社会精神文明
C．使医务人员和医疗卫生单位树立良好的形象，从而促进社会精神文明
D．使医务人员和医疗卫生单位树立良好的形象，在医疗竞争中立于不败之地
E．使医务人员养成良好的医德品质和人格，使医疗卫生单位形成良好的医德医风，存医疗竞争中立于不败之地

排除桥式起重机小车运行机构启动时车身扭摆的故障的方法有（）。

A.慢速启动；

B.调整轮压、调整主动轮；

C.慢速启动、调整主动轮；

D.慢速启动、调整轮压、调整主动轮。