of which/whom

[Tan Elaine]

My grandparents had five children, of which/whom my father was the third.

Should I use 'which' or 'whom'?

Thanks in advance.

 

[emsr2d2]

My grandparents had five children, of which/whom my father was the third.

Should I use 'which' or 'whom'?

Thanks in advance.

Whom.

As a general rule:

Who/whom/whose etc refer to people.
Which refers to things.