A factor is a term that is multiplying another term.
The expression (6z^4 - 4) is adding to another term.
The expression (-4 + 9(y^3 + 9)) is also adding to another term.
The expression (y^3 + 9) is multiplying the number 9.
And the expression 9(y^3 + 9) is adding to another term.
Therefore the correct option is C.
