A convention is codified into law ______.A．because

单项选择题

The main feature of a convention--a pattern of behavior that is customary, expected, and self-enforced--is that, out of a host of conceivable choices, only one is actually used. This fact also explains why conventions are needed: they resolve problems of indeterminacy in interactions that have multiple equilibria. Indeed, from a forma[ point of view, we may define a convention as an equilibrium that everyone expects in interactions that have more than one equilibrium.
The economic significance of conventions is that they reduce transaction costs. Imagine the inconvenience if, whenever two vehicles approached one another, the drivers had to get out and negotiate which side of the road to take. Or consider the cost of having to switch freight from one type of railroad to another whenever a journey involves both a wide-gauge and a narrow-gauge railroad line. This was a common circumstance in the nineteenth century and not unknown in the late twentieth: until recently, Australia had different rail gauges in the states of South Wales and Victoria, forcing a mechanical switch for all trains bound between Sydney and Melbourne.
Conventions are also a notable feature of legal contracts. People rely on standard leases, wills, purchasing agreements, construction contracts and the like, because it is less costly to fill in the blanks of a standard contract than to Create one from scratch. Even more important, such agreements are backed up by legal precedent, so the signatories have even greater confidence that, their terms are enforceable.
We may discern two ways in which conventions become established. One is by central authority. Following the French Revolution, for example, it was decreed that horse-drawn carriages in Paris should keep to the right. The previous custom had been for carriages to keep left and for pedestrians to keep right, facing the oncoming traffic. Changing the custom was symbolic of the new order going on the left had become politically incorrect because it was identified with the privileged classes; going on the right was the habit of the common many and therefore-more "democratic."
In Britain, by contrast there seems to have been no single defining event that gave rise to the dominant convention of left-handed driving. Rather, it grew up by local custom, spreading from one region to another. This is the second mechanism by which conventions become established: the gradual accretion of precedent. The two mechanisms are not mutually exclusive, of course. Society often converges on a convention first by an informal process of accretion; later it is codified into law to regulate exceptions. In many countries, rules of the road were not legislated until the nineteenth century, but by this time the law was merely reiterating what had already become established custom.
The surprising fact is that until the end of the eighteenth century, the dominant convention was for horse-drawn carriages to keep to the left. This situation obtained in Great Britain, France, Sweden, Portugal, Austria, Hungary, Bohemia and parts of Italy. A chain of historical accidents--Napoleon adopting the new convention for his armies and imposing this convention in occupied countries; Portugal sharing a common border with occupied Spain; Austria, Hungary and Bohemian Czechoslovakia falling under German rule; Italy having elected a "modern" leader under a king--gradually tipped the balance.

A convention is codified into law ______.

A．because not all follow the convention.
B．to keep legislators busy.
C．to reduce transportation costs.
D．for all of the above reasons.