题目

欲鉴别氢气、氧气、二氧化碳、甲烷4种气体，现有一盒火柴，还需要用到下列实验用品中的 A．装有蒸馏水的烧杯         B．装有无色酚酞的烧杯 C．冷而干燥的烧杯           D．内壁蘸有澄清石灰水的烧杯 答案：D _   we left the building that I realized  how dangerous the situation had been.     A．Only then    B．It was only then C．Only after  D．It was only after