题目

He has made some spelling mistakes in his composition; ___ they are not serious. A. however         B. everywhere        C. therefore     D. since 答案：A下列原子序数表示的两种元素，能形成共价化合物的是①11和10    ②6和16    ③1和17   ④12和8A.①②          B. ②③         C. ①③          D. ③④