题目

According to the air traffic rules, you ______ switch off your mobile phone before bearding. A. may                        B. can                          C. would                     D. should 答案：D请用化学符号表示（1）2个碳酸根离子________；（2）氧化铝中氧元素的化合价________；（3）2个金原子________；（4）有二氧化碳生成的置换反应________；（5）有氧气生成的分解反应________．