Read the following paragraph and underline the topic sentence. 　

Read the following paragraph and underline the topic sentence. 　Paraphrasing，as well as quotation，is an essential skill in writing research papers. Paraphrasing is used for different purposes. Some paraphrases will be designated to support already existing evidence. Others will reinforce argumentation against evidence. Still others will help to develop existing arguments and provide back-up for any conclusion drawn in the course of writing. Depending on the function，paraphrases will be introduced in accordance with their unique context.
【正确答案】：Paraphrasing is used for different purposes.
【题目解析】：主题句（topic sentence）是表达段落主题的句子。它用以概括段落大意，要求全段其他文字都围绕它展开。主题句通常放在段落的开端，其特点是开门见山地摆出问题，然后加以详细说明。其作用是使文章的结构更清晰，更具说服力，便于读者迅速地把握主题和想象全段的内容。主题句可以放在段中起到承上启下的作用，或放在段尾起概括全段的作用。本题中的段落谈的是Paraphrasing 的different purposes，全段都是围绕这一主题展开的。所以，Paraphrasing is used for different purposes.是主题句。