[Grammar] Them/whom

[Proficiencystudent]

"As many as 60 million people, a quarter of ______ were killed in Asia and the Pacific, died."

I think I should use 'them' in here, but the answer is 'whom'. Can somebody explain why?
Thanks.

 

[teechar]

If you omit "were", then you can use "them." Otherwise, you need a relative pronoun (whom).