下列事件中，属于必然事件的是(     )

阅读理解

      Peru(秘鲁)is the fifth largest Latin American country with a people of 14, 430, 000. Almost half of these people live in the city areas(地区). Around Lima, there were 20% of the total (总数) population in 1961. The population here grew faster than anywhere else and in another 35 years reached 1/3 of the total population.

      About 45% of the people are Indians and 50% are white or mixed（混合人种）and the others are Asians, Negroes(黑人)or people from other countries.

      In island areas, only about half of the people can read and write, but at the places near the sea, 2/3 of the people can read and write.

王大爷家里种的麦子有部分倒伏了，伴有叶色枯黄。你认为可使用下列化肥中的（）

科研人员分别给三只大白鼠注射了Ⅰ、Ⅱ、Ⅲ三种激素，下表表示的是注射相应激素一段时间后出现的生理反应，据表判断Ⅰ、Ⅱ、Ⅲ三种激素的化学名称分别是（　　）

读下图，图中盐度由低到高排列正确的是：(    )