题目

用化学式或化学符号填空: ①地壳中含量最多的元素            ②二氧化硫          ③2个氧分子      ④碳酸钙中钙元素的化合价为+2价               ⑤2个硫酸根离子        答案：.O  SO2  2O2  CaCO3  2SO42-It suddenly __________ to me that they went there by bike.A. happened B. occurred C. struck D. appeared