简要叙述光时域反射仪（OTDR：OpticalTimeDomainReflectometer）测试原理？

光时域反射法（OTDR）Optical time domain reflection method (OTDR) 基本原理：光源发出的光在沿光纤向前传输的过程中产生后向散射，后向散射光强在向后传播过程中随着距离增长而按一定规律衰减，在光速不变的情况下，距离与时间成正比。因此，根据探测器探测到的后向散射光强及其到达探测器的时间，就可以知道沿光纤路径上任一点的初始后向散射光强。Basic principle: the light emitted by the light source will generate backscattering in the process of forward transmission along the optical fiber, and the intensity of backscattered light will decrease in a certain rule as the distance increases in the process of backward propagation. Under the condition that the speed of light remains unchanged, the distance is proportional to time. Therefore, the initial backscattered light intensity at any point along the fiber path can be known based on the backscattered light intensity detected by the detector and the time it took to reach the detector. 光的后向散射包括瑞利散射、喇曼散射和布里渊散射3种形式。The backscattering of light includes Rayleigh scattering, Raman scattering and brillouin scattering.

A、光时域反射(OTDR)法Optical time domain reflection (OTDR) method

B、干涉法interferometry

C、布拉格光栅法（FBG）Bragg grating method (FBG)

D、波长扫描法和连续波调频法Wavelength scanning and continuous wave frequency modulation

A、时域

B、频域

C、复数域

D、离散域

A、1个

B、2个

C、3个

D、4个

Your network consists of a single Active Directory domain that contains two domain controllers. Bothdomain controllers run Windows Server 2003 Service Pack 2 (SP2).Auditing of successful account logon events is enabled on all computers in the domain.You need to identify the last time a specific user logged on to the domain.What should you do? （）

A. Examine the System Event Log on the user’s computer.

B. Examine the System Event Log on both domain controllers.

C. Examine the Security Event Log on both domain controllers.

D. Examine the Application Event Log on the user’s computer.

The departure time is 11:00 am and the( ) time of arrival is 19:15 local time in the evening.

A、根据 format格式定义，解析字符串t，返回 struct_time类型时间变量

B、将 struct_time对象变量t转换为时间戳

C、将当前程序挂起secs秒，挂起即暂停执行

D、返回一个代表时间的精确浮点数，两次或多次调用，其差值用来计时

A、ru.time.x

B、timep.x

C、(*timep).x

D、time->x