Why Were Manual Wage Workers Known as "Blue Collar" Workers

Why were manual wage workers known as "blue collar" workers?

A) Union members wore blue scarfs to recognize each other.
B) Workers on the shop-floor wore blue shirts and overalls.
C) The white shirts of workers took on a blue sheen in the dim factory lights of the late nineteenth century.
D) The term hinted at the "blue" feeling most workers had about their social status.