In biology, the biotic factors pertain to the living components of Earth while the abiotic factors are those non-living chemicals and physical factors. The only choice that is not an interaction between biotic and abiotic factors is the third choice, "C. male elk fighting over a female".
The circulatory system - hemoglobin in the blood transports oxygen :)
The Griffith's experiment, the Avery-MacLeod-McCarty experiment, and the Hershey–Chase experiments were the set of experiments that established DNA as the key hereditary molecule. The Avery-MacLeod-McCarty experiment was an extension to the Griffith's experiment. The heat killed virulent S strain cells of the Griffith's experiment were lysed to form a supernatant containing a mix of RNA, DNA, proteins and lipids from the cell. The supernatent was equally divided into 3 parts after the removal of the lipids. The 3 parts were respectively treated with an RNAase to degrade the RNA, DNAase to degrade the DNA and proteinase to degrade the proteins. The treated supernatant was then added into the culture containing the non-virulent R cells. In case of the supernatant treated with the DNAse, no transformation of R cells into S cells occurred. The transformation of R cells to S cells occurred in the proteinase and the RNAse cases. This indicated that DNA was the hereditary molecule and not protein or RNA.

Answer:
The "short tail" dominant allele is easier to eliminate by selective breeding.
Explanation:
The only way for a recessive allele to be expressed (be visible) is when it appears as recessive homozygotic. These means the organisms need to have 2 copies of the gene. Selective breeding is based on the characteristics that one can see, so if the organism shows the "dilute" phenotype you can keep reproducing this individuals and get rid of the dominant allele.
On the other hand if you have a population with the dominant phenotype, you discard all the ones that have a recessive trait and you breed the dominant phenotype you could still get individuals with the recessive phenotype and individuals that express the dominant phenotype but are heterozygous.