Obama made _______ history by being elected as ________ first black president of the US.

A．不填；the

B．a; the

C．a; a

D．the; a

区分有粒与无粒白细胞的主要依据是（）。

A、细胞大小不同

B、细胞有无分叶

C、细胞内有无特殊颗粒

D、细胞内有无嗜天青颗粒