题目

The students _______ busily when Miss Brown went to get a book she _______ in the office. A. had written; left      B. were writing; has left  C. had written; had left   D. were writing; had left 答案：D 解析: 略（2013?虹口区二模）科学研究发现，金星大气层的成分之一是三氧化二碳（C2O3），下列说法正确的是（　　）A．C2O3的摩尔质量是72gB．1molC2O3中含有1molO3C．C2O3的物质的量是指C2O3的质量D．1molC2O3所含氧原子约比1molCO2多6.02×1023个