The growth of switching has (135) a new generation of network management tools that help (136) cope with the challenges, (137) them to correlate device alarms in order to (138) pinpoint root causes, or to monitor service levels without depending on IP subnet schemes. These new tools have become essential to coping with the primary (139) effect of any treatment that significantly increases network flexibility: added complexity.

139（）

A.technicians 

B.workers 

C.salesmen 

D.professors

假设X是任意总体，μ=E(X)和σ²=D(X)存在，X₁，X₂，…，X_n是来自X的简单随机样本，

是样本均值，记
求：

。