Which pair of the following compounds are constitutional isomers?

Question

Quizplus · Accepted Answer

Constitutional isomers are compounds with the same molecular formula but different connectivity of atoms. 
I: H3C-CH2-CH2-CH3 
III: CH3-CH2-CH2-CH3 
I and III have the same molecular formula (C4H10) but different connectivity, making them constitutional isomers. 
II: H2C=CH-CH2-CH3 
IV: H3C-CH2-C≡CH 
II and IV have different molecular formulas, so they are not constitutional isomers. 
C and IV have the same connectivity (both are straight chains), so they are not constitutional isomers.

Which Pair of the Following Compounds Are Constitutional Isomers