题目

It was the first time that she ______ such a heavy rain. A . had witnessed B . has witnessed C . should witness D . witnessed 答案：A下列判断中一定正确的是（　　） A、若A2+2D-═2A-+D2，则还原性：D-＞A-B、若R2+和M+的电子层结构相同，则碱性：ROH＞MOHC、若X2、Y2都是气态单质，且酸性HX＞HY，元素原子得电子能力X＞YD、若M、N是同主族元素，且原子序数：M＞N，则元素原子得电子能力：M＞N