1. consider two clear species as defined by the biological species concept: a. fertile hybrids can be quite common if they are selected against intensively b. fertile hybrids can be continuously present but relatively uncommon c. hybrids can be quite common but infertile d. all of the above e. none of the above

The correct answer is option d. "All of the above".

Explanation:

A species is defined as a group of similar living organisms, constituting the basic unit of taxonomic rank of an organism. In Biology an hybrid is considered a species. It doesn't matter if the hybrid comes from combining the qualities of two organisms of different breeds, varieties or species Infertile hybrids such as mules and ligers are considered species as well.

b. read the phase at the intersection of any given temperature and pressure.