Once there was an old farmer. He had a horse and the horse was almost as old as himself. One morning he set out with his young son to sell the horse before it died. The father and the son walked because the farmer did not want the horse to be too tired . They met two men on the road. They said, "Why are you walking, farmer? You have a horse. It's a long way to get to the market. "The farmer knew that this was true, so he rode on the horse while his son walked. Then they met two old ladies. "What are you doing up there, farmer? Can't you see how tired your son is? "So the farmer got down, and let his son ride on it. Twenty minutes later, three old men stopped them. One said, "Why are you walking, farmer? Get up; it's too hot for an old man like you to walk today. "So the farmer got up behind his son and they went on riding. Some time later, a young woman passed them. "Why aren't you walking?"She asked, "It isn't far to the market. Give your poor old horse a rest. " So the farmer and his son got down once again. It is a fact that you cannot please all the people all the time. The farmer got up behind his son because   _  .
the old man said nothing. the three old men stopped them on the road he did not know why he was walking his son could not ride the horse by himself