Quiz: Bitmask Subsets

To iterate all subsets of a bitmask $m$, you use: $s = m$; while $s > 0$: process($s$); $s = (s-1) \& m$. Why does this work?

1. A.It generates numbers in decreasing order
2. B.It uses binary counting
3. C.$s-1$ is always a subset of $m$
4. D.$(s-1) \& m$ gives the next smaller subset of $m$