I often laugh when I see my grandma learning pop songs. But she says, " One is

never _____ old to learn. "              [ ]

A. too                      

B. so              

C. very                      

D. quite

网络优化中，进行网络测试的目的何在？以GSM为例，要对其上行、下行及全网各自进行哪些测试？