题目

The school is no longer what it was 20 years ago, ______ it was so poorly equipped. A. when            B. which        C. since         D. where   答案：A1.    when=at that time,修饰前面的20 years ago。  请用化学符号填空：（1）2个汞原子2Hg2Hg   （2）4个钙离子4Ca2+4Ca2+   （3）氧化铝中铝元素的化合价为+3价+3Al2O3+3Al2O3     （4）两个氮气分子2N22N2   （5）氯化亚铁FeCl2FeCl2．