求助prep3 ，关于过去完成时时态

3  Less than 35 years after the release of African honeybees outside Sao Paulo, Brazil, their descendants, popularly known as killer bees, had migrated as far north as southern Texas.

(A) Less than 35 years after the release of African honeybees outside Sao Paulo, Brazil,

(B) In less than 35 years since releasing African honeybees outside Sao Paulo, Brazil,

(C) In less than the 35 years since African honeybees had been released outside Sao Paulo, Brazil,

(D) It took less than 35 years from the release of African honeybees outside Sao Paulo, Brazil, when

(E) It took less than the 35 years after the time that African honeybees were released outside Sao Paulo, Brazil, and then

本题正确答案是A,详解上已给出很详细的答案，

这里我有个小小的疑问，关于migrate的时态，从何判断是过去完成时呢？过去的过去？还是强调过去的事对于过去的影响？

以我所见，gmat考试，对过去完成时用法很严格。

请教大牛，谢谢

为什么没人回我，大家看看，时态gmat中很重要哦

不是NN，个人看法，关键在after，看Longman上的解释：