设计变更时，设计单位应及时下达设计变更通知单，必要时附图，并逐条注明应修改图纸的图号。( )
参考答案：对
细胞代谢产生的各种酸使pH值下降，细胞培养液中的碳酸氢钠通过什么原理维持细胞培养液中pH值的正常( )
A．产钠离子B．产生O2C．产生CO2D．产生阴、阳离子E．作为平衡盐
As oil pipe line connections were broken,（）quantity of oil was caused to spill onto the wharf and into the dock water.
A.a not knowing
B.an unknowing
C.an unknown
D.a not known
