The predictability of our mortality rates is something that has long puzzled social scientists. After all, there is no natural reason why 2, 500 people should accidentally shoot themselves each year or why 7, 000 should drown or 55, 000 die in their cars. No one establishes a quota for each type of death. It just happens that they follow a consistent pattern year after year. A few years ago a Canadian psychologist named Gerald Wilde became interested in this phenomenon. He noticed that mortality rates for violent and accidental deaths throughout the western world have remained oddly static throughout the whole of the century, despite all the technological advances and increases in safety standards that have happened in that time. Wilde developed an intriguing theory called "risk homeostasis". According to this theory, people instinctively live with a certain level of risk. When something is made safer, people will get around the measure in some way to reassert the original level of danger. If, for instance, they are required to wear seat belts, they will feel safer and thus will drive a little faster and a little more recklessly, thereby statistically canceling out the benefits that the seat belt confers. Other studies have shown that where an intersection is made safer, the accident rate invariably falls there but rises to a compensating level elsewhere along the same stretch of road. It appears, then, that we have an innate need for danger. In all events, it is becoming clearer and clearer to scientists that the factors influencing our lifespan are far more subtle and complex than had been previously thought. It now appears that if you wish to live a long life, it isn't simply a matter of adhering to certain precautions ..., eating the right foods, not smoking, driving with care. You must also have the right attitude. Scientists at the Duke University Medical Center made a 15-year study of 500 persons personalities and found, somewhat to their surprise, that people with a suspicious or mistrustful nature die prematurely far more often than people with a sunny disposition. Looking on the bright side, it seems, can add years to your life span.

单选题 What social scientists have long felt puzzled about?

单选题 In his research, Gerald Wilde finds that technological advances and increases in safety standards ______.

【正确答案】 D

【答案解析】解析：细节题。短文第二段第二句指出mortality rates for violent and accidental deaths throughout the western world have remained oddly static throughout the whole of the century，despite all the technological advances and increases in safety standards that have happened in that time．即虽然技术取得了进步，安全标准也有了提高，但是西方国家的暴力和意外死亡率却保持着奇怪的恒定，由此可知，技术进步和安全标准的提高并未降低死亡人数。故选项D正确。

单选题 According to the theory of "risk homeostasis", some traffic accidents result from______.

单选题 By saying "... statistically canceling out the benefits that the seat belt confers"(Para. 2), the author means______.

【正确答案】 C

【答案解析】解析：细节题。由题干可找到定位句If, for instance，they are required to wear seat belts，they will feel safer and thus will drive a little faster and a little more recklessly，thereby statistically canceling out the benefits that the seat belt confers．意为当司机被要求系上安全带后，他们就会感到安全了，从而驾驶速度会快一些，也更鲁莽一些。这就造成其他方面的危险增大，抵消了安全带带来的安全。故选项C正确。

单选题 Which of the following may contribute to a longer lifespan?

【正确答案】 C

【答案解析】解析：细节题。短文最后一段指出a sunny disposition，looking on the bright side可以有助于长寿，结合选项可知C正确。A对别人信任不要怀疑是对最后一段people with a suspicious or mistrustful nature die prematurely far more often than people with a sunny disposition．的误解：此句意为“疑心重和不信任他人的人比乐观豁达的人早死”，并不是长寿的秘诀；B吃低脂肪食物和开车小心是对eating the right foods，not smoking，driving with care．的误解，吃正确的食物并不代表吃低脂肪食物；D看事物美好的一面并发展平衡的风险水平，其中后半句developing a balanced level of risk文中没有涉及，故排除。