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. Yet high rates of straying can be problematic because misdirected fish may interbreed with the existing stock to such a degree that any local adaptations that are present become diluted. Straying rates remain relatively low when environmental conditions are stable, but can increase dramatically when streams suffer severe disturbance。
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
请问答案确实是d吗？为什么？b哪里错了?
什么叫should the stream recover????
万分感谢！！！ | 
