[单项选择]At all ages and at all stages of life, fear presents a problem to almost everyone. "We are largely the playthings of our fears," wrote the British author Horace Walpole many years ago. "To one, fear of the dark; to another, of physical pain; to a third, of public ridicule; to a fourth, of poverty; to a fifth, of loneliness—for all of us our particular creature waits in a hidden place." Fear is often a useful emotion. When you become frightened, many physical changes occur within your body. Your heartbeat and responses quicken; your pupils expand to admit more light; large quantities of energy-producing adrenaline (肾上激素) are poured into your bloodstream. Confronted with a fire or accident, fear can fuel life-saving flight (逃离). Similarly, when a danger is psychological rather than physical, fear can force you to take self-protective measures. It is only when fear is disproportional to the danger at hand that it becomes a problem. Some people are simply more vulnerable to fear than others. A visit to the newborn nursery of any large hospital will demonstrate that, from the moment of their births, a few fortunate infants respond calmly to sudden fear-producing situations such as a loudly slammed door. Yet a neighbor in the next bed may cry out with profound fright. From birth, he or she is more prone to learn fearful responses because he or she has inherited a tendency to be more sensitive. Further, psychologists know that our early experiences and relationships strongly shape and determine our later fears. A young man named Bill, for example, grew up with a father who regarded each adversity as a temporary obstacle to be overcome with imagination and courage. Using his father as a model, Bill came to welcome adventure and to trust his own ability to solve problem. Phil"s dad, however, spent most of his time trying to protect himself and his family. Afraid to risk the insecurity of a job change, he remained unhappy in one position. He avoided long vacations because "the car might break down." Growing up in such a home, Phil naturally learned to become fearful and tense.Fear can be a useful emotion to us because it can ______.
A. stimulate many physical changes within our body
B. quicken our heartbeat and responses
C. pour large quantities of adrenaline into our bloodstream
D. help us respond quickly to danger and protect ourselves