三年级有5个班，每班28人，如果每人捐2本书，三年级同学一共可捐多少本书？
28×5×2=280（本）
答：三年级一共可捐书280本。
— I hear that you failed the exam again. Why was that?
— I would really appreciate _________ you didn't mention this. [ ]
A. you if
B. it if
C. it when
D. that
下列观点符合质量守恒定律的是
A．一定条件下，SO2和O2生成SO3，反应前后分子总数不变
B．镁燃烧后剩余残渣的质量减轻了
C．8gCO完全燃烧生成8g CO2
D．某物质在空气中燃烧只生成CO2和H2O，则该物质一定含有碳、氢、氧元素
