Answer:
b. The frequency of the lap94 allele at different sites results from an interaction between selection and gene flow.
Step-by-step explanation:
In the southwest portion of the Sound, salinity is relatively low, and selection against the lap94 allele is strong. Moving toward the northeast and into the open ocean, where salinity is relatively high, selection favors a high frequency of the lap94 allele. However, because mussel larvae disperse long distances, gene flow prevents the lap94 allele from becoming fixed in the open ocean or from declining to zero in the southwestern portion of Long Island Sound.