What do the following pairs of structures represent?

The total number of structural isomers for the molecular formula $C_5H_{10}$ (including cyclic and acyclic isomers) is:

Which of the following pairs of compounds are positional isomers?

In which of the following is functional group isomerism $NOT$ possible?

Given below are two statements :
Statement $(I) :$ $CH_3CH=CHCH_3$ and $cyclobutane$ are isomeric compounds.
Statement $(II) :$ $CH_3CH_2CH_2CH_2NH_2$ and $(CH_3CH_2)_2NH$ are functional group isomers.
In the light of the above statements,choose the correct answer from the options given below :

Methoxy methane $(CH_3-O-CH_3)$ and ethanol $(CH_3-CH_2-OH)$ are: