One Reason Why, in the Last Four Decades, the Number

One reason why, in the last four decades, the number of new auto makers in the world has been very small compared to the past is that

A) the automobile cannot be improved upon in any way by new producers.
B) new auto makers cannot obtain necessary inputs to produce new cars.
C) governments restrict who can produce automobiles.
D) new producers cannot match the economies of scale of existing auto makers.