Determine whether each of these sets is countable or uncountable. For those that are countably infinite, exhibit a one-to-one correspondence between the set of positive integers and that set. a) integers not divisible by 3. b) integers divisible by 5 but not by 7. c) the real numbers with decimal representations consisting of all 1s. d) the real numbers with decimal representations of all 1s or 9s.