Desertification is a growing world problem, and nowadays the United Nations and its member countries _________hard to combat it. A.are working B.has worked C.work D.worked 答案:A两种元素可以形成AB2型共价化合物的是A.无中子的原子与最外层有6个电子的短周期元素的原子B.核电荷数分别为12和17的元素C.ⅣA族和ⅥA族原子半径最小的元素D.最高正价都为奇数的两种短周期元素