Corruption is problem in countries across the world, with developing countries often believed to be more serious, although there is no evidence to suggest this is case.
A. a; the
B. the; a
C. 不填; the
D. a; a
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(答案→)A. a; the
B. the; a
C. 不填; the
D. a; a
解析：句意：腐败是一个全世界国家的问题，发展中国家被认为是更加严重，虽然没有证据表明情况是这样。第一空是泛指“一个问题”，第二空是特指上文的情况，所以选A。