贫血与下列哪项有关（　　）

①红细胞数量过少   ②白细胞数量过少   ③血小板数量过少   ④血红蛋白数量过少．

古诗词阅读 ，回答相关问题。            
                                                                                     使至塞上
                                                                                        王维
                                                                    单车欲问边，属国过居延。
                                                                    征蓬出汉塞，归雁入胡天。
                                                                    大漠孤烟直，长河落日圆。
                                                                    萧关逢候骑，都护在燕然。