The residues ("tailings") of mines often contain such high concentrations of toxic metals (e.g., copper, lead) that most plants are unable to grow on them. However, some hardy species (e.g., certain grasses) are able to spread from the surrounding uncontaminated soil into such areas. These plants evolve resistance to the toxic metals, while their ability to grow on uncontaminated soil decreases. Because grasses are wind pollinated, breeding between the resistant and nonresistant populations continues, but offspring of crosses between the two populations are intermediate and have low fitness. This is an example of

A) stabilizing selection.
B) directional selection.
C) frequency-dependent selection.
D) disruptive selection.
E) underdominance.