Which explanation best accounts for the differences in resistance to Dutch Elm Disease among elm trees?

The correct explanation reflects the concept of genetic variation, which is fundamental in understanding how certain species, including elm trees, can exhibit differing levels of resistance to diseases such as Dutch Elm Disease. Some trees possess specific genetic variations that confer advantages, such as enhanced immune responses or the ability to withstand the pathogen that causes the disease.

This inherited resistance results from natural selection acting on the genetic diversity within the population. Trees that are genetically predisposed to resist the disease are more likely to survive and reproduce, passing those beneficial traits on to subsequent generations. This process contributes to the overall population's adaptability and resilience against threats like Dutch Elm Disease.

In contrast, the other options do not adequately explain the observed differences in resistance. Acquiring resistance through experience does not apply to trees as they do not learn or adapt behaviors; rather, they rely on their genetic makeup. The assertion that all trees are equally resistant fails to recognize the existence of variations among individuals. Lastly, environmental conditions certainly play a role in disease spread and tree health but do not account for the genetic basis of resistance. Therefore, the focus on beneficial genetic variations provides a clear and scientifically supported explanation for the differences in resistance among elm trees.