Why are the 1300 nm and 1550 nm regions of the fiber spectrum primarily used for optical communications？（） [" They are both regions of lowest loss."," They are both regions of low reflections."," They are both regions of low index of refraction."," They are both regions of low chromatic dispersion."," They are both regions of low Polarization Mode Dispersion （PMD）."]
清净分散剂
What is the maximum number of CTI ports for a Cisco Unified Contact Center Express 4.0 Standard deployment? （） ["150 ","200 ","300 ","varies based on the number of expansion servers used ","varies based on the number of applications configured&e"]
当二极管的两端加上反向电压，加到某临界电压（）以后，反向电流开始急剧（），若不加限制，就会（）该现象称为（）。
下面关于塑料门窗框固定的方法正确的是（）。 ["混凝土墙洞口采用膨胀螺钉固定，严禁采用射钉固定","砖墙洞口应用膨胀螺钉固定，不得固定在砖缝处，并严禁用射钉固定","轻质砌块或加气混凝土洞口可在预埋混凝土块上用射钉固定，严禁用膨胀螺钉固定","当门窗与墙体固定时，应先固定边框，后固定上框"]
简述脊柱牵引的生理效应及其影响因素。
