A researcher is interested to test the hypothesis that a 100 base pair stretch of a given gene is ...

A researcher is interested to test the hypothesis that a 100 base pair stretch of a given gene is under selection.
 
  She sequences this gene region in 30 individuals, each of two populations. She finds that within either popula-tion, there are 33 synonymous sites of which 10 are segregating for different alleles in the population. In this same sequence, she observes 29 sites within-populations that possess variable alleles at nonsynonymous sites. In contrast, when she compares substitutions across species, she finds that of the 33 synonymous sites only five show signs of a substitution between species. Only 15 nonsynonymous substitutions were observed between species. This suggests that the gene is:
  a. under purifying selection.
  b. under nearly neutral selection.
  c. under positive selection.
  d. under directional selection.
  e. This cannot be determined based on the information provided.