Mosquitoes in tropical regions can carry and transmit the malaria parasite to humans. Mosquitoes used to be killed in these regions by spraying them with insecticide DDT. Mosquitoes that were sensitive to DDT are now not killed by DDT and grow and multiply even in the presence of DDT. How can you explain this phenomenon?

Options:
A) All mosquitoes must have undergone a mutation for DDT resistance and produced a similar kind of resistant offspring which evolved into a DDT-resistant population.
B) Some mosquitoes may have undergone a mutation for DDT resistance and produced similar kinds of resistant offspring, which would have been selected by nature.
C) All mosquitoes must have solely increased their tolerance level to DDT and ultimately would become completely resistant to it.
D) The phenomenon is not explainable based on the given information.