单选题：根据以下资料，回答题。　　As one approaches some crossroads, one comes to

根据以下资料，回答题。 　　As one approaches some crossroads, one comes to a sign which says that drivers have to stop when they come to the main road ahead．At other crossroads, drivers have to go slow, but they do not actually have to stop (unless, of course, there is something coming along the main road); and at still others, they do not have either to stop or to go slow, because they are themselves on the main road.
　　Mr．Williams, who was always a very careful driver, was driving home from work one evening when he came to a crossroads．It had a "Slow" sign, so he slowed down when he came to the main road, looked both ways to see that nothing was coming, and then drove across without stopping completely.
　　At once he heard a police whistle, so he pulled in to the side of the road and stopped．A policeman walked over to him with a notebook and pencil in his hand and said, "You didn't stop at the crossing."
　　"But the sign there doesn't say 'Stop'," answered Mr Williams．"It just says ‘Slow’, and 1 did go slow."
　　The policeman looked around him, and a look of surprise came over his face．Then he put his notebook and pencil away, scratched his head and said, "Well, I'll be blowed! I am in the wrong street!"
At a crossroads, you reduce your car's speed when A． there is s stop sign
B． there is a slow sign
C． you are on the main road
D． you are on the crossroad