已知A，B，C点在球O的球面上，∠BAC=90°，AB=AC=2．球心O到平面ABC的距离为1，则球O的表面积为（　　）

                                                                                               金陵驿二首（其一）
                                                                                                     文天祥
                                                                                    草合离宫转夕晖，孤云飘泊复何依！
                                                                                    山河风景元无异，城郭人民半已非。
                                                                                    满地芦花和我老，旧家燕子傍谁飞？
                                                                                    从今别却江南路，化作啼鹃带血归。

通常肉类食物的价格要比粮食和蔬菜高．从生态学的观点来看，这是由于（  ）