单选题 .  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.11.  What is it that social scientists have long felt puzzled about? ______