4 Researchers know that the prevalence of COVID in population A is 5%, and that the prevalence of COVID in population B is 2%. A researcher flips a fair coin, if it lands on heads then he selects one person at random from population A and tests for COVID. If the coin lands on tails he follows the same protocol using a randomly selected person from population B. If the COVID test came back positive (and assuming that the test is 100% accurate), what is the probability that the coin landed in tails?