一同卵双胞胎兄弟俩长大后身体胖瘦差别很大，其主要原因是

用肉眼观察幼根时，所看见的白色“绒毛”部分是（　　）

阅读下面的诗歌，完成小题。
                                                                                                                          绝句二首（其一）
                                                                                                                                      杜甫
                                                                                                                   迟日江山丽，春风花草香。

                                                                                                                   泥融飞燕子，沙暖睡鸳鸯。
      【注】此诗作于杜甫草堂。