6. An athlete suspected of using steroids is given two tests that operate independently of each other. Test A has probability 0.9 of being positive if steroids are used, and test B has probability 0.8 of being positive if steroids have been used. What is the probability that at least one of the two tests is positive? (a) 0.98 (b) 0.02 (c) 0.72 (d) 0.38