Identify the pairs of compounds which are functional group isomers.
$I$. $CH_3-CH_2-CH_2-CH_2-OH$
$II$. $CH_3-CH_2-CH(OH)-CH_3$
$III$. $(CH_3)_3C-OH$
$IV$. $CH_3-CH(CH_3)-CH_2-OH$
$V$. $CH_3-CH_2-O-CH_2-CH_3$
$VI$. $CH_3-O-CH_2-CH_2-CH_3$
$VII$. $CH_3-O-CH(CH_3)_2$

(A) Functional group isomers are compounds that have the same molecular formula but different functional groups.
$I, II, III, IV$ are alcohols (functional group $-OH$).
$V, VI, VII$ are ethers (functional group $-O-$).
Any pair consisting of one alcohol and one ether will be functional group isomers (e.g.,$I$ and $V$).
$V, VI, VII$ are ethers and exhibit metamery due to different alkyl groups attached to the oxygen atom.