题目

He did well in the examination. He _____ hard at his lessons..       A. must have worked     B. can have worked     C. will have worked      D. has worked 答案：A下列物质其中属于氧化物的是（　　）A．氯气（Cl2）B．二氧化氯（ClO2）C．臭氧（O3）D．次氯酸钙 Ca（ClO）2