PREP语法题时态问题求解

stellapan95

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,  
DIt 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  


A选项是正确答案，但搞不懂为什么后面要用过去完成时had migrated?
一般不都是before一个时间点,才用had done吗
这里说 35 years after the release of African honeybees,后面直接用一般过去时不就可以了吗？