One among the following pairs of compounds is not isomers:

Question

$(a)\;(CH_3)_2CH - O -C_2H_5\;and \;CH_3(CH_2)_2 -O -C_2H_5 \\(b)\;CH_3CH_2CO_2H \;and\; CH_3CO_2CH_3 \\ (c)\;CH_3COCH_3 \;and\; CH_3CH_2CH_2OH \\ (d)\;CH_3CH_2NO_2 \;and\; NH_2CH_2COOH $