A:who was it that killed
B:who it was that killed
C:it was who killed
D:who was it killed
命题“∀x∈R,x2≥0”的否定为 ( )
①二氧化硅是制造光导纤维的重要原料
②硅、碳原子最外层都有4个电子,故SiO2的物理性质与CO2类似
③SiO2既能和氢氧化钠溶液反应也能和氢氟酸反应,是两性氧化物
④将CO2通入Na2SiO3溶液中有胶状沉淀生成,说明碳酸比硅酸酸性强.
“到巴西旅游,我终生难忘!”李明从巴西回来逢人便说。他为什么对这次旅游记忆如此深刻呢?下面是他的回答,其中有一句完全是编造的,请你找出来。