题目

   Over thirty thousand years ago people from northern Asia（亚洲）went to America. Today, we call these people Indians（印第安人）.     The Indians went to America because the weather began to change. Northern Asia became very cold. Everything froze. They had to move or die. How did the first Indians go to America? They walked!     Later Columbus found the New World in 1492. At first, only a few Europeans（欧洲人）followed（跟随）. They traveled to America in boats. For the next three hundred years, about 500,000 people went there. Then the number grew very qu如图，AB=AC，CD⊥AB于点D，BE⊥AC于点E，BE与CD相交于点O（1）图中有几对全等的直角三角形？请你选择其中一对进行证明；（2）连接OA、BC，试判断直线OA、BC的关系并说明理由．