下列实验方法不能达到实验目的是（　　）

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地球公转时围绕的旋转中心是（　　）

如图表示肺泡内的气体交换过程．据图回答问题．

等差数列$\left\{a_n\right\}$的公差为$d$，前$n$项和为$s_n$，当首项$a_1$和$d$变化时，$a_2+a_8+a_{11}$是一个定值，则下列各数中也为定值的是                    （  ）

短文改错
假定英语课上英语老师要求同桌之间交换修改作文，请你修改你同桌写的以下作文。文中共有10处语言错误，每句中至多有两处。错误涉及一个单词的增加、删除或修改。
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，并在该词下面写出修改后的词。
注意：1. 每处错误及其修改均限一词。2. 只允许修改10处，多者（从第11处起）不计分。
    Today I visited the Smiths — my first time visit to a American family. They live in a small peacefully town. It was very kind for them to meet me at the railway station and drove me to their home. The Smiths did their best to make me feel at home. They offered myself coffee and other drinks. We have a good time talking and laughing together. They eager to know everything about China and asked me lots of question. In fact, they are planning to visit China in next year.