请教GWD-TN-1里的一道RC题

WW200730010062

请问在GWD-TN-1里的Q10
GWD1-Q10:
It can be inferred from the passage that the occasional failure of some salmon to return to their natal streams in order to spawn provides a mechanism by which
A.    pristine streams that are near polluted streams become polluted themselves
B.    the particular adaptations of a polluted stream’s salmon population can be preserved without dilution
C.    the number of salmon in pristine habitats decreases relative to the number in polluted streams
D.    an environmentally degraded stream could be recolonized by new salmon populations should the stream recover
E.    the extinction of the salmon populations that spawn in polluted streams is accelerated
答案为什么是选D？
想了好久都不明白，望牛人们赐教，不甚感激！！

TeamCancer

文中有原话

terryhan

语句：Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulated should the fish there disappear.