If you have the rows indices, then it's just a matter of simple indexing. To extract the row numbers contained in the variable B, then for any arbitrary matrix it's simply:
If your A has only one column, as in your case, then you don't even need the colon, so:
However, if your B is the result of find, then you don't even need to use find nor generate B. You can use logical indexing instead. e.g.:
Using logical indexing will be (negligibly for arrays of your size) faster than using find + normal indexing.