What is the correct $IUPAC$ name of the following structure?

The $IUPAC$ name of the compound $CH_3-C(=O)-CH_2-C(CH_3)_2-CN$ is:

The $IUPAC$ name of $(CH_3)_2CHCH(CH_3)_2$ is:

In the given compound,the number of $2^{\circ}$ carbon atom/s is. . . . . .
$CH_3-C(CH_3)(H)-CH(H)-C(CH_3)(H)-CH_3$

$IUPAC$ name of the given structure is:

Which of the following $IUPAC$ names is correct for the structure:
$CH_3-CH(CH_3)-CH_2-CH_2-CH_2-CH(CH_2-CH_2-CH_3)-CH_2-CH_3$?
$(i)$ $2-$methyl$-6-$ethylnonane
$(ii)$ $6-$ethyl$-2-$methylnonane
$(iii)$ $4-$ethyl$-8-$methylnonane