Which of the Following Most Accurately Describes the Preservation of Duplicate

Which of the following most accurately describes the preservation of duplicate genes via the subfunctionalization model?

A) The ancestral gene performs two different functions, and eventually the two duplicate copies evolve so that one copy performs one function, and the other copy performs the other function; both copies are therefore required for the organism to have optimal fitness.
B) In order to be favored by natural selection and therefore to increase in frequency, the duplicated copies must evolve so that together they perform the original gene's functions better than the original gene, giving organisms with subfunctionalized genes a fitness advantage.
C) Subfunctionalization is most commonly a defense against mobile genetic elements. Because subfunctionalized genes separate two functions from an ancestral gene into two separate genes, a mobile genetic element insertion can damage only one of the two functions, not both; therefore, subfunctionalization is favored by natural selection in genomes with a high load of mobile genetic elements.
D) All that matters for the subfunctionalization model is that together the duplicated genes perform the original gene's function(s) the same or better; even if one gene copy becomes nonfunctional, this still counts as subfunctionalization so long as the other copy performs the original function.