问题补充:
Barrack Obama President of the United States since .
A. became a
B. has become a
C. has turned
D. has been
答案:
A. became a
B. has become a
C. has turned
(答案→)D. has been
解析:Barrack Obama自从一直是美国的总统。和since 连用,用现在完成时,而且因为是一段时间,所以不能和短暂动词become连用,所以选D。