Answer:
B) the mobility of their hosts
Explanation:
A pathogen that attacks an oak trees and a pathogen that attacks humans doesn't really have that big of a difference, but on the other side we can see that the pathogen that attacks the oak trees is spreading out much less and over much smaller territory than the pathogen attacking humans. The main reason behind this is the mobility of the host of the pathogen and where the host of it lives. In the case with the oak trees, the pathogen will be migrating very slowly over smaller distance because there's only certain places where the oak trees live, and also they are not mobile, and the pathogen will be able to spread out only through their seeds and cones which is a slow process. On the other hand, the pathogen attacking humans will manage to spread out very quickly over very large area because there are humans living in lot of places, their numbers are high, and their mobility is easy and quick over large distances, so the pathogen will travel with its host easily for hundreds or thousands of kilometers and then spread out in another place far away.
Xylem tissue makes up a root organ
Small population size is your answer mate.
