PT 21 S2 Q19

Spectroscopic analysis has revealed the existence of frozen nitrogen, methane, ... ...

The astronomers' argument relies on which one of the following assumptions?
(A) There is no more frozen nitrogen on the surface of Pluto than there is either frozen carbon monoxide or methane.
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in NP: Practice Set 3 - Mix (25 Q)