根据材料内容，选择正确答案。   

    Some people in the West like to stay at home during their weekends. They work in the garden, visit their friends, read books or watch television. Many families take their lunch to a park or somewhere far from the city. They like to eat under some trees or near a nice lake. If they live near the sea, they often go to the seaside. There they can fish, swim or enjoy the sunshine.

    Some people in the West like travelling. They think travelling and outdoor activities are interesting and exciting. They work hard to save money, but their main purpose is to travel.

    Most Western students often make use of their weekends to camp. There are thousands of interesting camp bases in Western countries. They can swim, go fishing, attend lectures, and take part in many other activities there. It can help them quest_ for the secrets of nature and teach them how to be self ­reliant(自立的).

①空气中的N₂在放电条件下与O₂直接化合，生成无色且不溶于水的一氧化氮气体；

②一氧化氮的密度比空气略大，不稳定，常温下就易与空气中的O₂反应，生成红棕色的二氧化氮气体；

③二氧化氮气体有毒，易与水反应生成硝酸(HNO₃)和一氧化氮；

④生成的硝酸随雨水洒到大地上，同土壤中的矿物质相互作用，生成可溶于水的硝酸盐。