Obama was elected first black president of the United States four years ago and has been

问题补充：

Obama was elected first black president of the United States four years ago and has been elected president again for the next four years.

A. the; the

B. the; X

C. X; the

D. a; a

答案：

A. the; the

(答案→)B. the; X

C. X; the

D. a; a

解析：

序数词前加the，the first black president第一位黑人总统。而第二空，职位前面省略the.故选B