在关系数据库中，对一个关系做投影操作后，新关系的元组个数将（ ）。
A． 等于原来关系中的元组个数
B． 大于原来关系中的元组个数
C． 小于原来关系中的元组个数
D． 不能确定元组个数
参考答案：A
解析：投影是从关系中指定若干个属性组成新的关系，是从列的角度进行的运算，相当于对关系进行垂直分解，与元组个数无关，产生的新关系元组个数与原来关系中的元组个数相等。
第三部分任务型阅读（满分10分） Stress is an uncomfortable feeling in the mind from any situation or thought that makes you feel frustrated, angry, or anxious. Stress if often accompanied by phy  sical symptoms, including twitching or trembling, muscle tension, headaches, sweating, dry mouth, difficulty swallowing, rapid or irregular heart rate, rapid breathing, fatigue, loss of temper, sleeping difficulties and nightmares, decreased concentration.Stress is a normal part of life. In small quantities, stress is good – it can motivate you and help you be more productive. However, too much stress, or a strong response to stress, is harmful. It can cause general poor health as well as specific physical or depression. Persistent and never-ceasing stress often leads to anxiety and unhealthy behaviors like overeating and abuse of alcohol or drugs. Certain drugs can lead to symptoms of stress due to side effects. Such drugs include caffeine, alcohol, nicotine, cocaine and diet pills. A poor diet – for example, low levels of vitamin B 12 – can also contribute to stress or anxiety. Performance stress is related to specific situations, like taking a test or making a presentation in public. Posttraumatic stress develops after an unforgettably shocking event like war, physical attacks, or a natural disaster. Any effective ways to cope with your stress or anxiety? One step is to make a list of what you think might be making you ‘stress out’: What do you worry about most? Is something constantly on your mind? Does anything in particular make you sad or depressed? Then, find someone you believe in who will listen to you. Often, just talking to a friend or loved one is all that is needed to relieve a  nxiety. Most communities also have support groups and hotlines that can help. Social workers, psychologists, and other mental health professionals may be needed for therapy and medication (疗法和打坐).Also, find healthy ways to deal with stress. For example, eat a well – balanced, healthy diet, don’t overeat, get enough sleep, exercise regularly, limit caffeine and alcohol. Don’t use nicotine, cocaine, or other recreational drugs. Learn and practice relaxation techniques like yoga and tai chi. Take breaks from work. Make sure to balance fun activities with your responsibilities. Spend time with people you enjoy. Find self – help books at your local library or bookstore. Title: Stress
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