Between 1630 and 1853, japan experienced a period of self-imposed isolation from outsiders. what geographical feature most influenced japan's isolation?

a.
japan is an island.
b.
japan has many mountains.
c.
japan has many rivers.
d.
japan has dense forests.
explain pls *

Japan has many mountains, thus, the outsiders could not easily enter Japan and Japanese could not leave the country. And, thus the self-isolation was helped by the presence of a mountain terrain. This policy was implemented by Togukawa, who insisted on preventing Japan from foreigners. 

It covered the territory of land across eurasia from northeastern europe to the pacific ocean.