题目

With all flights________, they had to come by bus. A . had canceled B . canceled C . have been canceled D . having canceled 答案：B完成下列反应的化学方程式．（1）实验室加热高锰酸钾制氧气的反应：2KMnO4  △  . K2MnO4+MnO2+O2↑2KMnO4  △  . K2MnO4+MnO2+O2↑．（2）二氧化碳和澄清的石灰水的反应：Ca（OH）2+CO2=CaCO3↓+H2OCa（OH）2+CO2=CaCO3↓+H2O．（3）碳还原氧化铜的反应：C+2CuO 高温 . 2Cu+CO2↑C+2CuO 高温 . 2Cu+CO2↑．