在△ABC中，面积S=a²﹣（b﹣c）²，则cosA=（　　）

阅读下面这首古诗，回答以下两题。
                                                                                                            夏 意（苏舜钦）
                                                                                                           别院深深夏席清，
                                                                                                           石榴开遍透窗明。
                                                                                                           树阴满地日当午，
                                                                                                           梦觉流莺时一声。
                                                                                             注：别院，即正院旁侧的小院。

下图反映了古代历史上的“种姓制度”，这一制度出现于（    ）