根据短文内容回答问题

Dear Mike，

      I'm so glad you're coming to China! Your mother，our foreign English teacher，told me you're worried about what you should and shouldn't do as a guest in China．Here's some advice．You will probably be invited to a big meal. You may find it earlier than you expect because Chinese people eat dinner much earlier than people in the West．There will be a lot of different dishes，not just three courses．You may find it difficult to use chopsticks if you haven't tried before．It's a good idea to get some practice．When the food is served，you should take time to look at it. The preparation takes a long time and everything will look beautiful．The plates will be placed in the middle of the table，for everyone to share．Take a little of everything and then have some more of the things you like．Chinese people like to talk about food，so it's a good topic of conversation．At the end of the meal，you will usually be served with fruit．Eat your fruit，thank your friends， and go．

      I hope this is helpful．E-mail me if you have any other questions． 

                                                                                                                              Yours，

                                                                                                                                   David Zhang

$\frac{15}{17}\times 8+\frac{15}{17}\times 7+\frac{15}{17}$ =（   ）

下列关于磁感线的说法中，正确的是（　　）

2010年6月5日是第38个世界环境日，世界环境日中国的主题为“低碳减排，绿色生活”．