| What exactly is a lie？ Is it anything we say which we know is untrue？ Or is it something more than that？ For example, suppose a friend wants to borrow some money from you. You say "I wish I could help you, but I am short of money myself." In fact, you are not short of money but your friend is in the habit of not paying his debts and you don't want to hurt his feelings by reminding him of this. Is this really a lie？ Professor Jerald Jellison of the University of southern California has made a scientific study of lying. According to him, women are better liars than men, particularly when telling a "white lie", such as when a woman at a party tells another woman that she likes her dress when she really thinks it's terrible. However, this is only one side of the story. Other researchers say that men are more likely to tell more serious lies, such as making a promise that they have no intention of carrying out. This is the kind of lie politicians and businessmen are supposed to be particularly skilled at: the lie from which the liar hopes to profit or gain in some way. Research has been done into the way people's behavior changes in a number of small, unimportant ways when they lie. It has been found that if they are sitting down at the time, they tend to move about in their chairs more than usual. To the trained observer they are saying "I wish I were somewhere else now". They also tend to touch certain parts of the face, particularly the nose. One explanation of this may be that lying causes a slight increase in blood pressure. The tip of the nose is very sensitive to changes and the increased pressure makes it itch. Another gesture that gives liars away is what the writer Decmond Morris in his bookMan Watching calls the "mouth cover". He says that there are several typical forms of this,such as covering part of the mouth with the fingers, touching the upper lip or putting a finger of the hand at one side off the mouth. Such a gesture can be understood as an unconscious (未察觉的) attempt on the part of the liar to stop himself from lying. Of course, such gestures as rubbing the nose or covering the mouth, moving about in a chair can not be taken as proof that the speaker is lying. They simply tend to happen more often in this situation. It is one gesture alone that gives the liar away but a whole number of things, and in particular the context (上下文) which the lie is told. | 
题目内容
（请给出正确答案）
简答题官方参考答案
（由简答题聘请的专业题库老师提供的解答）
. Then the random variable 
follows an exponential distribution with PDF 
.
A likely reason for the difference in the slopes of the demand curves is that:
