In the Context of NPSs, What Are Positional Isomers

In the context of NPSs, what are positional isomers?

A) stereoisomers of existing substances
B) new substances that differ from a previously identified one only in where a substituent is located
C) compounds that fall in a different category or position on the controlled substances list
D) compounds with reversed positions of the molecular ion in an EI-MS spectrum
E) none of the above