You're in a department store and you see a couple of attractive young women looking at a sweater．You listen to their conversation：

    “I can't believe it－Lorenzo Bertolla! They are almost impossible to find．Isn't it beautiful? And it's a lot cheaper than the one Sara bought in Rome．”

    They leave and you go over to see this incredible sweater．It's nice and the price is right．You've never heard of Lorenzo Bertolla, but those girls looked really stylish．They must know．So, you buy it．You never realize that those young women are employees of an advertising agency．They are actually paid to go from store to store, talking loudly about Lorenzo Bertolla clothes．

    Every day we notice what people are wearing, driving and eating．If the person looks cool, the product seems cool, too．This is the secret of undercover marketing．Companies from Ford to Nike are starting to use it．

    Undercover marketing is important because it reaches people that don't pay attention to traditional advertising．This is particularly true of the MTV generation－consumers between the age of 18 and （3）It's a golden group．They have a lot of money to spend, but they don't trust ads．

    So advertising agencies hire young actors to “perform” in bars and other places where young adults go．Some people might call this practice misleading, but marketing executive Jonathan Ressler calls it creative．“Look at traditional advertising．Its effectiveness is decreasing．”It is true, because everyone knows an ad is trying to persuade you to buy something．However, you don't know when a conversation you overhear is just a performance．

在原子内部，核外电子绕原子核运动会形成一种环形电流，该环形电流产生的磁场使物质微粒（原子）的两侧相当于两个磁极．若图中箭头表示的是电子绕原子核运动的方向，则环形电流的左侧应为____ （选填“N”或“S”）极．

下列各组数中不能作为直角三角形的三边长的是（     ）

购买单价为m元的钢笔5支和单价为n元的铅笔7支，应付款____ 元．