A:was kept
B:were kept
C:has been kept
D:have been kept
医学上常用双氧水(H2O2)来清洗创口和局部抗菌。某同学为了测定一瓶医用双氧水溶液的溶质质量分数,取该双氧水68g放入烧杯中,然后加入2g二氧化锰,完全反应后,称得烧杯内剩余物质的总质量为66.8g。请回答下列问题:
--I . By the time I got outside, the school bus had already left.
早上你在校门口遇到你的英语老师,你应该向老师说_______打招呼。