患者女性，22岁。以头昏乏力、月经量增多半年入院。平时无挑食现象。曾因腹泻服用过"氯霉素"。体检：贫血貌，四肢皮肤可见散在瘀点、瘀斑，心肺无异常，腹软，肝、脾肋下未及。实验室检查：RBC2．2×10⁹／L，Hb68g／L，WBC3．5×10⁹／L，N37％，L60％，M3％，PLT46×10⁹／L。

再生障碍性贫血的发生机制是（）.

A.造血干细胞异常

B.免疫机制异常

C.造血微环境缺陷

D.细胞因子γ干扰素升高

E.遗传倾向

F.电离辐射

G.药物和化学因素

While I __________ to the English programes, mother __________ outside.[ ]

A. listened; was washing

B. was listening;washed

C. was listening; was washing

D. listened; was washing