NLTK中收录了许多语料，以下问题基于选择合适的语料库构建一个小型项目的思路。例如其中有两份语料分别名为“product_reviews_1”和“product_reviews_2”，假设我们来观察前一份语料，打开NLTK存放语料的目录（“nltk_data\corpora\product_reviews_1”）我们发现其中有一个文件readme.txt，文件中说明本语料与评论产品某个特征（如相机本身或相机镜头）的情感（褒义或贬义）有关，打开文件“Canon_G3.txt”，显然这是关于佳能相机G3本身和其各类特征的评价，形如：picture[+3]##the highest optical zoom pictures are perfect .menu[+1],button[+1]##the menus are easy to navigate and the buttons are easy to use .camera[+3]##it is a fantastic camera and well worth the price .flash photo[-3],noise[-2]##well flash photos are never great , and there was still a lot of noise .##you can even take a picture of yourself while looking at the lcd .dial[-1]##* main dial is not backlit .观察你本地相应目录下的文件（包括readme.txt）或以上的语料片段，回答以下问题。【情感分析第1题】语料中每一条评论句是以以下哪种形式开始的？