黏油罐车下卸阀需进行气密性试验,手把置于45°位置时,另一端堵住,进行600KPa的压力试验,需保压()不得漏泄。
A.30s
B.60s
C.90s
D.120s
参考答案:B
____ seems strange to us is ______ she can sing so many English songs beautifully.
A.It; that
B.That; how
C.It; Why
D.What; that
什么是X射线管的阴极特性?