问题补充：
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
答案：
(答案→)A. to have discovered
B. to have been discovered
C. to be discovered
D. having been discovered
解析：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。
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