On a cold winter day, a fox ____  mother bear that he would teach her how to fish. As she____ to learn, he took her to a hole in the ice, and told her to put her tail down in the water. He told her to keep the tail there for____ .  Then____ she pulled the tail out of the water, she would find fish around the tail. The bear was foolish enough to believe____ the fox said. She sat for a long time with her tail in the water. She was ____ the fish. But when she____  to pull her tail lout of the water, she found that it was frozen in the ice. She asked the fox to pull her out. But he laughed to himself and ____ . so she called father bear to help her. Father bear came. He pulled hard and ____ got her out, but a part of her tail was in the ice. That is____ people find the bear’s tail so short.

下列做法属于在传播途径中控制噪声的是（　　）

四边形ABCD中，AD=3，AB=4，BC=12，CD=13，∠BAD=90°，则△BDC为____三角形．