Where Is the Mistake in the Following Proof

Multiple Choice

Where is the mistake in the following proof?
1) (?x) (Fx · Ax)
2.(?x) (Fx · Ox)
3.? (?x) (Ax · Ox)
4.Fb · Ab (1, E.I.)
5.Fb · Ob (2, E.I.)
6.Ab · Fb (4, Com.)
7.Ab (6, Simp.)
8.Ob · Fb (5, Com.)
9.Ob (8, Simp.)
10.Ab · Ob (7, 9, Conj.)
11.(?x) (Ax · Ox) (10, E.G.)

A) line 4
B) line 8
C) line 5
D) line 9