The main difference between natural and artificial selection is that human beings______.

A．the selection by a farmer of the best milk-producing cows for breeding
B．a breeder's allowing only the fastest horses to reproduce
C．the selection for reproduction of the best egg-laying chickens by the farmer
D．an increase in the number of giraffes with long necks because of a decline in the number of low-lying plants used for feeding

A．Labor restrictions prevent the two from handling the same traffic and have prevented competition from developing.
B．Railroads carry different commodities and thus are not in direct competition with water transport.
C．They sometimes compete with one another.
D．Railroads are never routed along the same routes as water transport, so the two do not compete.