Determine whether each of these sets is countable or un- countable. 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 con- sisting of all 1s d. the real numbers with decimal representations of all 1s or 9s