Gmat prep - confused

[s_raizada]

OA is A

Could someone explain the use of past perfect 'Had migrated' here? Where is simple past sentence to go with past perfect?

[VP_Jim]

Remember that past perfect is used to show that some event in the past happened before another event in the past. While it's practically always used with a simple past verb, it doesn't have to be.

In this case, we want to show that the bees' arrival in Texas took place before the 35th anniversary of their release.

Hope this helps!

[s_raizada]

Thanks Jim!

Now I know this rule.

[sukrant26]

Am sorry, I didnt get the explanation .

Is the answer A.