Passage Two A hundred years ago, the game we now call football did not exist. American football started during a game between two colleges. The teams had got together to play what they called "football", but each team played by different roles. One team played what we now call soccer. The other played what we now call rugby (橄榄球) . Both games had been invented a thousand years before. In the first kind of football game ever played, all the men from one village tried to kick a ball into another village. The men of the second village tried to kick the ball into the first. Hundreds of people joined in, running everywhere, running crops and knocking down fences. In time, people agreed on some rules to keep order, but many rules were left open to change. Different rules developed in different places. When the two colleges met to play football, each followed its own rules. They mixed the games together and invented a new game. A hundred years later we call that game American football. In what ways do you suppose the game we know now will have changed in another hundred years What does the passage mainly discuss（）

A. The changes of soccer.
B. The rules of rugby.
C. Different village players.
D. The birth of American football.