题目

“m＜0”是“函数f（x）=m+log2x（x≥1）存在零点”的（ ） A．充分不必要条件       B．必要不充分条件 C．充要条件        D．既不充分又不必要条件 答案： AIt is sometimes reported that strange flying objects have been seen high up in the sky. People call these strange objects Unidentified(不明的)Flying Objects(UFOs). These UFOs caused a lot of interest. Some of the reports about them are difficult to believe.Some have been explained in scientific ways. Others have never been explained.Here is an account of UFOs from a report.Dennis Bardens,who wrote an article,tells us that he once saw a UFO himself.He was having dinner one night with a friend near Tripoli(的黎波里),Libya(利比亚),and after dinner the two men walked across a yard to his room to look at some papers.Bardens noticed that the moon looked strange that night.“It wasn't the moon,”was the reply.They watched the light and saw that it was moving.It seemed to be an object rather like a big plate.In some ways it looked like smoke with a light on it,but it wasn't smoke because it didn't change its shape.It was moving in a regular way.That is to say,it didn't go suddenly faster or more slowly,and it didn't go higher or lower.So the two men decided that there must be men in it.When it came nearer to Libya,Bardens said that he felt eyes looking at him.He was being watched in the darkness.When it was nearer,they noticed that the bottom parts were brighter than the top.The bottom was yellow and the top was red.Then the object suddenly turned away and left Libya,moving very fast.On a later day Bardens talked to other people about it,and he found some who had seen it.They all described(描述)it in the same way.Could they all the mistaken?60.The UFOs are______.A.flying high up in the sky       B.strange flying objectsC.difficult to believe            D.very interested61.A UFO was seen one night______.A.when they were walking across a yard to Bardens' roomB.while they were looking at the lightC.when Bardens noticed the moon appearing strangeD.while they were having dinner62.The UFO looked like______.A.smoke                                   B.lightC.a moving object                         D.a large plate63.When the UFO came nearer to Libya,______.A.it disappeared all at onceB.it suddenly turned away and leftC.it was watched in the darknessD.Bardens was looking at it