Consider the following incomplete proof of the sequent $(notnot{a}^{{?}^{?}}b.$

$($b$)$

premise

Ne ${?}_{1}1$

a

not$2$

b

$N{e}_{2}1$

$...$

Choose from the options to complete the proof.

$5\mathrm{.}7$a assumption

$6.\_\_\_\_3,5$

$7.$a $-|$e $6$

not b assumption

$9.\_\_\_\_4,8$

$$

$\_$e $9$

$a^{{?}^{?}}$ b b

Ai $7,10$

$$a

assumption

$-1-$

$-|\_$i $3,5$

a

e $6$

Ai $7,4$

This sequent cannot be proven since it does not hold.

$$

assumption

$-\_$

$-\_$ i $3,5$

$(a^{{?}^{?}}b)$

$-\_$e $6$