Why Did Office Work Become the Fastest-Growing Sector of the Female

Why did office work become the fastest-growing sector of the female labor force by 1900?

A) Women were thought to be especially suited to typing on typewriters.
B) Office work offered women the only opportunity to earn wages equal to those of men.
C) Office work required little education and offered better wages than factory work.
D) Office work was the only industry that had a union that accepted women.