Which set of molecules are not considered constitutional isomers?

Question

Quizplus · Accepted Answer

Constitutional isomers are molecules with the same molecular formula but different connectivity of atoms. 
I and V are constitutional isomers since they have the same molecular formula but different arrangements of atoms. 
II and IV are constitutional isomers since they have the same molecular formula but different branching patterns. 
III, however, is not a constitutional isomer since it is the exact same molecule as I, just in a rotated orientation.

Which Set of Molecules Are Not Considered Constitutional Isomers