— 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
如图为细胞中ATP与ADP相互转化示意图,相关叙述正确的是( )
图中分别为人体呼吸系统、泌尿系统及循环系统部分结构示意图
某地1000千克化肥的市场价格如下:CO(NH2)21080元;NH4NO3810元;NH4HCO3330元。请计算: