题目

Steve was a seventh grader, a big boy, looking more like a teenager than a 12­year­old. Yet, he went unnoticed—he had been ____ every examination since first grade—until Miss Wilma appeared.In the middle of the first semester of school, the entire seventh grade was ____ for basic skills.“You all did pretty well，” Miss Wilma told the class after going over the ____ ， “except for one boy. And it ____ my heart to tell you this, but...” she ____， “the smartest boy in the seventh grade is failing my class.” Steve ____ his eyes and carefully examined his fingertips.After that下列物质中，前者属于氧化物，后者属于混合物的是（　　）A. 蒸馏水、石灰水 B. 高锰酸钾、河水C. 空气、生铁 D. 天然气、石灰石