Trait B is likely to have arisen earlier. In asexual reproduction, cell divisions results in creation of similar cells with identical copies of DNA. Therefore, in such species, a trait may come into existence in some members due to sudden mutation induced by the environmental factors. This newly induced trait in some members of species is then inheritance by future generations of such organism on replication over a period of time. So, it is but natural for trait B to exist earlier than trait A, as 60% of population with trait B is likely to have been replicating that trait for a longer period than 10% of population with trait A.