Andris,
I hope we are assuming that A, B and D together do not provide all the world’s needs, leaving C with nothing to do, for that is an uinrealistic assumption, as AE emphasizes.
So that C focuses on what there is a scarcity of, that either A, B, or D do not do best [w.lo.g it’s furniture], and gets in there making furniture for A, freeing up A’s time for cars.
Here’s a bit of Mises on the subject:
Ricardo’s first aim in expounding this law was to refute an objection
raised against freedom of international trade. The protectionist asks: What
under free trade will be the fate of a country in which the conditions for any
kind of production are less favorable than in all other countries?
Now, in a
world in which there is free mobility not only for products, but no less for
capital goods and for labor, a country so little suited for production would
cease to be used as the seat of any human industry. If people fare better
without exploiting the—comparatively unsatisfactory—physical conditions
of production offered by this country, they will not settle here and will leave
it as uninhabited as the polar regions, the tundras and the deserts.
But Ricardo
deals with a world whose conditions are determined by settlement in earlier
days, a world in which capital goods and labor are bound to the soil by
definite institutions. In such a milieu free trade, i.e., the free mobility of
commodities only, cannot bring about a state of affairs in which capital and
labor are distributed on the surface of the earth according to the better or
poorer physical opportunities afforded to the productivity of labor. Here the
law of comparative cost comes into operation.
Each country turns toward
those branches of production for which its conditions offer comparatively,
although not absolutely, the most favorable opportunities. For the inhabi-
tants of a country it is more advantageous to abstain from the exploitation
of some opportunities which—absolutely and technologically—are more
propitious and to import commodities produced abroad under conditions
which—absolutely and technologically—are less favorable than the unused
domestic resources.
The case is analogous to that of a surgeon who finds it
convenient to employ for the cleaning of the operating-room and the
instruments a man whom he excels in this performance also and to devote
himself exclusively to surgery, in which his superiority is higher.