A Main Motivating Factor for the Increase in Dual-Earner Couples

A main motivating factor for the increase in dual-earner couples is that:

A) women's incomes are now more similar to men's, compared to their incomes in the past.
B) child care is more available and more affordable.
C) couples are more likely to remain unmarried, and thus need greater independent financial security.
D) couples tend to have larger families than they did in the past.