Looping over every subset of a mask

• A

  4^n, because each of the 2^n masks has up to 2^n submasks

• B

  3^n, because each item is in the submask, in the rest, or in neither

• C

  2^n, because each submask is visited exactly once overall

• D

  n * 2^n, the same as looping over items for each mask