[originaltext]　　(18) Americans are afraid of crime and take many precautious to

　　(18) Americans are afraid of crime and take many precautious to keep from being victims. (16)Unfortunately, the people who are generally the most friendly and outgoing, out of a need to protect themselves, act and appear, unfriendly. and hostile to strangers. Though smiling is such a natural human trait, Americans have learned that to smile at a stranger is to invite danger. (16)They are daily exposed to news stories of victims who have been tortured, raped, murdered, and robbed by strangers. It is a logical reaction to view all strangers as potential criminals. With this idea in mind, visitors should always be mindful that Americans are overly cautious about whom they talk with and whom they allow within their personal space. An official from the US Department of State was once talking with a group of foreign travelers in Washington D. C. and his instructions were graphic and to the point. (17) He warned, "Each American has this imaginary bubble that surrounds him or her and extends about twenty inches in all directions. At all costs avoid getting too close to that bubble. " His advice clearly points out to visitors that (18) people are afraid of anyone who gets too close and threatens their personal safety. Visitors should be aware that out of fear, Americans may react violently to what may be an innocent smile, stare or pat on the back.
　　16. Why are Americans hostile towards strangers?
　　17. What does the official from the US Department of State suggest?
　　18. What can we infer about Americans from the passage?