suramya26
By the end of the Apollo program, twelve Americans
have walked on the moon.
This was the question
In the answer key it was provided that instead of
have walked it should be
had walked.
Couldn't we write
would have walked on the moon???
would have walked , if the Apollo program is still on-going and would end.
had walked, if Apollo program ended at the time this sentence was written.
Given that Apollo program got completed more than 3 decades ago, the question assumes that everyone is aware of that fact.