Two competing hypotheses to account for the increase in the number of hox genes from the last common ancestor of bilaterians to the last common ancestor of insects and vertebrates are: (1) a single duplication of the entire four-gene cluster, followed by the loss of one gene, and (2) three independent duplications of individual hox genes. to prefer the first hypothesis on the basis of parsimony requires the assumption that

The duplication of a cluster of four Hox genes is equally likely as the duplication of a single Hox gene.

Explanation:

This comes from the definition of the parsimony principle, which is the action of choosing the simplest scientific explanation fitting the given evidence, meaning that all other elements are equal when evaluating the hypothesis. In biology the best hypothesis is the one that requires the fewest evolutionary changes.

Since three independent duplications of an individual Hox gene is in  evolutionary terms, more complex than a single duplication or a cluster, then that hypothesis would be the most parsimonious.

i think that d. wind is the answer.   i missed the question when i answered a. currents.

the oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle. this blood then enters the left atrium, which pumps it through the mitral valve into the left ventricle. from the left ventricle, the blood passes through the aortic valve to the aorta.

deoxygenated blood leaves through the right ventricle through the pulmonary artery. from the right atrium, the blood is pumped through the tricuspid valve (or right atrioventricular valve), into the right ventricle.