食品包装中常常涉及到化学知识的运用：

（1）市场上销售的香肠、盐水鸭、榨菜、豆腐干等食品，常采用真空包装．真空包装的目的是除去空气，使大多数微生物因缺少______而受到抑制，停止繁殖，同时防止食品发生缓慢______而变质．

（2）茶叶、肉松、膨化食品等常采用真空充气包装，即将食品装入包装袋，抽出包装袋内空气，再充入防止食品变质的气体，然后封口．真空充气包装能使食品保持原有的色、香、味及营养价值，防止食品受压而破碎变形．

提出问题：充入食品包装袋中的气体是什么？

猜想与验证：可能是氮气．

实验初步验证的方法及现象：______．

充入的气体还可能是：①______；②______…

小题1:But still there is a danger that grows every year．Airliners get larger．Some airplanes can hold over 300 passengers．And the air itself becomes more and more crowded．If one giant airliner crashed into another in mid-air, 600 lives could be lost．

小题2:．Air traffic controllers tell the pilot exactly when to turn, when to climb, and when to come down．The air traffic controllers around a busy airport like London-Heathrow may handle 2500 planes a day．Not all of them actually land at the airport．Any plane that flies near the airport comes under the orders of the controllers there．小题3:

Recently such a disaster almost happened．小题4:One, with 69 passengers, had come from Toronto, and the other, with 176 passengers, from Chicago, An air traffic controller noticed on his radar screen that the two planes were too close to each other．He ordered one to turn to the right and to climb．But he made a mistake．He ordered the wrong plane to do this．Fifteen seconds later it flew directly in front of the second plane．They avoided each other by the smallest part of a second．The distance between them was less than that of a large swimming pool.小题5:.

A．Even a small mistake on their part could cause a disaster．

B．Two large jets were flying towards the airport．

C．Nowadays people like traveling more by air than by car．

D．Today, air travel is far safer than driving a car on a bus motor-way． E． This is an example of the danger that grows every year．

F．  In a word, air travel is more dangerous, we should choose others．

G． From the moment an airliner takes off to the moment it lands, every movement is watched on radar screen．