One Hundred Years Ago,in North America and Europe It Was

One hundred years ago,in North America and Europe it was commonplace for middle class households to employ housemaids.As you know,this is quite rare today.Most such households now have dishwashers,washing machines and dryers.What is an explanation of this change?

A) Over long periods of time,capital is always replaced by labour.
B) Households substituted away from a factor whose price was falling (labour) and toward a factor whose price was rising (capital) .
C) The marginal product of labour was falling over time while the marginal product of capital was rising.
D) Households substituted away from an increasingly expensive factor (labour) and toward an increasingly inexpensive factor (capital) .
E) Over long periods of time,labour is always replaced by capital.