By the end of the Apollo program

[yaswanthr]

Sentence correction guide (4th edition) , Chapter # 7, problem set # 8 (page # 117).

Question: Following sentence contains one or more underlined sections. If an underlined section contains no errors, mark it as CORRECT. Otherwise, write down a correct version of the underlined section.

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

Answer key explanation from guide : Have walked (present perfect tense) should be had walked (past perfect tense). The past perfect is required because the twelve Americans did their walking before the end of the Apollo program. Here the phrase end of the Apollo program functions much like a specific date in the past.

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" ?

[tim]

Did the strategy guide state or imply that using simple past was wrong? The section you quoted indicated that past perfect would be correct (possibly even ideal) but did not necessarily rule out the simple past. I wouldn't rule out simple past here, although the GMAT probably wouldn't use it in the correct answer and they almost certainly will NOT test you on the difference between past and past perfect in a context such as this.