PREP上一道语法题困惑

Nineteenth-century doctors thought that alcohol,when ingested, passed through the stomach into the bloodstream and was broken down only by the liver; recentstudies, however, have led them to conclude that the stomachbreaks down up to 20percent of ingestedalcohol before it reachesthe liver.

A.       have led themto conclude  

B.       have led to their conclusion  

C.       have led doctorsto conclude

D.       lead to doctors concluding  

E.       lead to[url=]the[/url]  doctors’ conclusion

能理解E选项中the的用法不对所以错，但是为什么正选是现在完成时时态呢...有些困惑