乙二醇精制塔压力由（）来调节。A、低压氮气 B、中压氮气 C、高压氮气 D、低压蒸汽

琼脂的加热方法有（）

A.温水中缓慢进浸泡融化

B.60℃恒温水浴中融化

C.电炉直接加热融化

D.琼脂加热器中融化

E.微波炉中融化

电压互感器二次侧为什么不允许短路？如果发生开路或短路分别应如何处理？

________he recites 10 words every day and his English has improved greatly.

A．On average

B．On purpose

C．And so on

D．On and on

I want to know how much _____ about the table manners around the world.         [ ]

A. do they know        

B. they know        

C. did they know      

D. have they knows

用计数式频率计测量频率，闸门时间（门控时间）为ls时，计数器读数为5400，这时的量化误差为多大？如将被测信号倍频4倍，又把闸门时间扩大到5倍，此时的量化误差为多大？