单项选择题
A.自由运行(Free Running) B.保持(Hold Over) C.锁定(Locked) D.跟踪(Trace)
A.1x10E-11 B.1x10E-10 C.1x10E-9 D.1x10E-8
A.TS0 B.TS1 C.TS16 D.TS30