—Mrs White always helps others _____ she is very busy.   —She is so kind.

A. but            B. because           C. although

根据《长途光缆波分复用（WDM）传输系统工程验收规范》，槽道和走线架安装时，列槽道和列走线架应成一条直线，水平偏差不得超过50mm。