对于各数互不相等的正数数组（i₁，i₂，…，i_n）（n是不小于2的正整数），如果在p＜q时有i_p＜i_q，则称“i_p与i_q”是该数组的一个“顺序”，一个数组中所有“顺序”的个数称为此数组的“顺序数”．例如，数组（2，4，3，1）中有顺序“2，4”、“2，3”，其“顺序数”等于2．若各数互不相等的正数数组（a₁，a₂，a₃，a₄，a₅）的“顺序数”是4，则（a₅，a₄，a₃，a₂，a₁）的“顺序数”是（　　）

钢铁在潮湿的空气中会发生吸氧腐蚀。以下说法中，正确的是（      ）

由¹H₂¹⁶O与²H₂¹⁸O所代表的物质中，共有　 ____种元素，　 ____种原子，¹⁶O、¹⁸O的关系 ____为

如图，在研究光的反射定律实验中，

    Spring Festival is our Chinese people's festival. There are twelve names. We call it the year of sheep, the year of dog… and the year of monkey. Before the day of the festival, people are very busy(忙于 ) shopping and cleaning their houses. On the Eve, there is a big family dinner. After dinner, all the family stay up late( 守夜 ) to welcome the new year. It is the middle of the night. We have some dumplings and watch many good TV programs( 节目). On the first day of the New Year, people wear their new clothes to go to their friends' houses. They say “ Happy Spring Festival” to each other. Children will get some money from their parents or other people. So people especially( 特别) children like Spring Festival very much.