重点讲解：
1. specialise in
专门从事；专门经营；专门研究；专攻；
eg. Most of the staff specialise in the care of children.
多数员工都掌握专业的儿童保育知识。
eg. As you know, we specialise in electronic games.
正如你们多了解的，我们专门经营电子游戏。
2. track down
追踪到；追查到；搜寻到；
eg. It took the police several days to track down their quarry.
警方用了几天时间追查到案中目标。
eg. He was determined to track down the cause of food spoilage.
他决心查出食品腐烂的原因。
3. upside down
倒置地；颠倒过来；
eg. The painting is upside down.
图画颠倒了。
eg. Tony had an upside-down map of Britain on his wall.
托尼的墙上倒挂了一张英国地图。