The Two Figures Represent an Asexual and a Sexual Population

The two figures represent an asexual and a sexual population. The letters indicate beneficial mutations. The shaded areas represent the frequency of particular combinations of mutations in the population over time. Why does it take longer for the beneficial allele combination of ABC to go to fixation in the asexual population than in the sexual population?

A) In the asexual population, new allelic combinations have to arise via de novo mutations.
B) There is no principal difference between asexual and sexual populations. In the sexual population shown, the favorable mutations just happen to be on different chromosomes.
C) The effective population size of both populations is the same.
D) Sexual populations tend to have higher mutation rates.