题目

下列化学用语中，正确的是 A．2个氧分子：2O                    B．一氧化碳：Co   C．3个钠离子：3Na+                   D．氧化钠：NaO 答案：C I recognized him ________ I saw him on the street one day after twenty year's painting． [　　] A．moment B．the minute C．while D．any time