said by School Marm :

The rule is, use "whom" when it's the object of the verb that can't be replaced by "he" or "she.

So, "To whom did you give the gift." Because "whom" is the object of the verb "to give" and you would also say, "I gave the gift to him or her."

Now in your case, "Comcast will have the brains to cross-reference who are business clients." Because you could also say, "cross-reference [if] he/she is [a] business client" or, "cross-reference if they are business clients."

Bottom line is, when the question is who/whom, try replacing the who part with he, she, they. If the sentence works with that replacement, use "who."

said by Datatechs :

Y'all dun screwed up da whole thang.

said by avd706 :

Whom did?

said by avd706 :

Brilliant!!!