问题补充：
The monument was built in honor of the explorer who was believed the river.
A. to have discovered
B. to have been discovered
C. to discover
D. having been discovered
答案：
(答案→)A. to have discovered
B. to have been discovered
C. to discover
D. having been discovered
解析：用不定式的完成式表示动作发生在(定语)从句谓语动作之前。
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