What is the $IUPAC$ name of Acrylic acid?

The structure of $4-$methylpent$-2-$ene is

Which of the following compounds is acetonitrile?

The $IUPAC$ name of $CH_3-CH(CH_3)-CO-CONHBr$ is

What is the $IUPAC$ name of the compound $CH_3-CH(C_2H_5)-CH(C_2H_5)-CH_3$?

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