Many yeast species of the genus Saccharomyces are found throughout the world in either wild or domesticated populations.  The domesticated populations are used by humans for industrial purposes (eg, food processing, pharmaceutical production).  Studies have shown that different Saccharomyces species, which may reproduce either asexually or sexually, have hybridized over time and that some of these hybridizations have resulted in the formation of new species.Studies of North American Saccharomyces yeast identified two new species thought to be the result of hybridization events.  One species (Sp3) is thought to have arisen approximately 10,000 years ago from the hybridization of two parental species (Sp1 and Sp2).  Another hybrid yeast species (Sp4) is thought to be the result of a more recent hybridization event between Sp1 and Sp3.In a study of reproduction among these four yeast species, scientists performed multiple sets of crosses among various species.  All sets of crosses produced a similar number of spores.  Next, they determined the percentage of resulting spores (ie, cells capable of dividing and developing into mature yeast cells) from each set of crosses that was viable.  The results are recorded in Table 1.Table 1  Viability of Spores Resulting From Crosses Among Various Saccharomyces Yeast SpeciesIn addition, the scientists confirmed and examined the growth (ie, asexual reproduction) of all four yeast species under several environmental conditions.  In most conditions, Sp3 exhibited a higher growth rate than Sp2 and a lower growth rate than Sp1 over time.  However, when grown in the presence of mannose, Sp3 exhibited a higher growth rate than both Sp2 and Sp1.  Sp4 exhibited the lowest growth rate among the species in most conditions.Eberlein C, Hénault M, Fijarczyk A, et al. Hybridization is a recurrent evolutionary stimulus in wild yeast speciation [published correction appears in Nat Commun. 2019 May 13;10(1):2199]. Nat Commun. 2019;10(1):923. Published 2019 Feb 25. doi:10.1038/s41467-019-08809-7 Question 28In the growth experiment conducted in the passage, which of the following best describes the independent variable?A.The different yeast species that were analyzedB.The growth rates of each yeast species over timeC.The total number of yeast cells for each species initially included in the experimentD.The environmental conditions to which yeast species were exposed