We can not live a modern life without traveling．The fastest way of traveling is by air．With a plane people can travel in one day to places which it took a month or more to get to a hundred years ago．
   Traveling by train is slower than by plane, but people can see the country when they are traveling through．Modern trains have comfortable seats and dinning-cars．They make even the longest journey（旅途） enjoyable．
   Some people prefer to travel by sea when possible．There are large luxury liners（客轮） or river boats．They are not as fast as trains or planes, but traveling by sea is a very pleasant way to spend a holiday．
   Many people like to travel by car．They can make their own timetable．They can travel hundreds of miles or only fifty miles a day, just as their wish．They can stop wherever they want to see something interesting or to enjoy a good meal at a good restaurant, or to spend the night at a hotel．That is why people choose traveling by car for pleasant trips, while they usually take trains or planes for business．

如图，在多面体ABCDEF中，四边形CDEF是正方形，AB∥CD，CD=2AB，G为DE的中点．

（1）求证：BG∥平面ADF；

（2）若CD=2，AB⊥BD，BD=BE，∠DBE=90°，求三棱锥A﹣BDF的体积．

（1）下列各组中的两种有机物，可能是相同的物质、同系物或同分异构体等，请判断它们之间的关系

①2﹣甲基丁烷和丁烷____ ．②1﹣已烯和环已烷____ ．

（2）支链只有一个乙基且式量最小的烷烃的结构简式____ ．

（3）写出乙醛溶液与足量的银氨溶液共热的离子方程式：____ ；

（4）写出1，3﹣丁二烯与溴单质发生1，4﹣加成的反应方程式____ ．

下图示意某国城镇人口和乡村人口的变化，读图回答下列各题