Historians have a difficult task, if not all facts have been discovered yet, with doing that?

They attempt to re-create a historic event to find what probably happened

Explanation:

When a historian cannot explain all the facts because there are not enough resources, in this case, he/she must see what was already found, wrote, researched about the event and think about the gaps. What could have happened there? Here, the imagination is important, because the historian will think about the probable events that occurred there, based on the sources he/she has. It's not wrong to proceed this way, because when other sources are found, historians can proof if the possibilities are correct or not.

the answer should be a. the sui empire, the tang empire extended from korea in the west to manchuria in the east.