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 discover
D. having been discovered
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(答案→)A. to have discovered
B. to have been discovered
C. to discover
D. having been discovered
解析：A 考察非谓语动词。句义：建造这个纪念碑是为了纪念那位被认为发现了那条大河的探险家。在be said/reperted/believed后面接不定式，此外固定搭配，发现大河是已经发生的动作，且探险家和发现之间是主动关系，因此使用不定式完成式表示主动形式。故A正确，