Quiz: Small-to-Large Complexity

Why does *small-to-large merging* lead to $O(n \log n)$ total work?

1. A.Because each merge is $O(1)$
2. B.Because each element moves into a set at least twice as large, so it moves at most $\log n$ times
3. C.Because you sort all sets before merging
4. D.Because you avoid merges when sizes are equal