题干
— How long has the train been away?
— Sorry. I don't know. By the time I got here, the train ________.
A：has left
B：was leaving
C：had been away
D：had left
答案(点此获取答案解析）
C
2NH3（g）；△H<0。10秒后达到平衡，c（NH3）为0.4mol/L。下列说法正确的是