Which two compounds are not isomers of each other?

A n-pentane and 2-methylbutane

B \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \) and \( \mathrm{CH}_{3} \mathrm{OCH}_{3} \)

C \( \mathrm{CH}_{3} \mathrm{COOH} \) and \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH} \)

D \( \mathrm{CH}_{3} \mathrm{COCH}_{3} \) and \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO} \)

Correct Answer: Option C