I would have come sooner but I ________ that they were waiting.

A．haven’t known

B．hadn’t known

C．don’t know

D．didn’t know

动物饲料中长期缺乏必需脂肪酸时，动物可出现（）

A.贫血

B.水肿

C.体内胆固醇量下降

D.不孕症

E.前列腺素合成增加