Why past perfect tense

[yaswanthr]

This is from the chapter # 7 of the sentence correction guide :

By the end of the Apollo program , twelve Americans have walked on the moon.

the guide states that have walked must be replaced with HAD WALKED.

My question : why can't we use simple past tense ?
By the end of the Apollo program , twelve Americans walked on the moon.
Here the phrase by the end of the program already clarifies the sequence of events, so why do we need to emphasize it again by using "HAD" ?

[RonPurewal]

Hi,

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Thanks.