I need help

Prove that the following subsets of (0, 1] are Borel-measurable. (a) any countable set (b) the set of irrational numbers (c) the Cantor set (Hint: rather than defining it in terms of ternary expansions, it's easier to use the equivalent definition of the Cantor set that involves sequentially removing the "middle-third" open intervals; see Wikipedia for example). (d) The set of numbers in (0, 1] whose decimal expansion does not contain 7