The monument was built in honor of the explorer who was believed＿＿the big river
A.to have discovered
B.to have been discovered
C.to be discovered
D.having been discovered
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A解析试题分析：考查不定式的完成式：句意：这座纪念碑被认为是为纪念发现这条河的探险家而建造的。be believed to have done为认为做过某事，因为探险家已经发现了这条河，用不定式的完成式，选A。考点：考查不定式的完成式