已知$a,b,c$分别$△ABC$三个内角A，B，C所对的边，若$\left(\frac{\overrightarrow{AB}}{\left|\overrightarrow{AB}\right|}+\frac{\overrightarrow{AC}}{\left|\overrightarrow{AC}\right|}\right)·\overrightarrow{BC}=0$且$△ABC$的面积$S_{△ABC}=\frac{a^2+c^2-b^2}{4}$，则三角形$△ABC$的形状是（    ）

读图回答。

            图一                                       图二

                                     图三

阅读下面材料，在空白处填入1个适当的单词或括号内单词的正确形式。

    Johnny, a lizard(蜥蜴), liked sunbathing every morning. One day, he felt so ____(relax) that he didn't notice some boys coming up behind him. The boys ____(catch) Johnny, and he could only escape by losing his tail.

    After the children left, Johnny came out to look ____his tail, but found no sign of it. Feeling sad, he made a ____(decide) to spend all his time looking for it. Time passed, and Johnny kept asking everyone whether ____(them) had seen his tail. One day, he asked a boy, who surprisedly replied, “Why do you need two tails?” Hearing this, Johnny turned back and saw a new, stronger tail. He ____(sudden) realized how silly it had been to waste so much time on such a problem.

    On his way home, he found his old tail on the roadside. ____it looked terrible, Johnny was still excited to have it back. He picked it up and was about to continue his journey when an idea hit him ____he shouldn't look at the past. He then dropped his old tail, ____(leave) with it all his past worries. All he took with him ____(be) thoughts of the future.

原始森林中，____分布于亚欧大陆北部、北美大陆北部；热带雨林主要分布非洲的____盆地、南美洲的____流域、东南亚等地区。我国林区主要分布在东北、____、西南部。