How many isomers are possible for a compound of molecular formula `C_4H_10` ?

A compound with molecular formula C_4H_6 may contain:

How many isomers of butene are possible ?

The compound having molecular formula C_4H_10O can show:

A compound A of molecular formula C_(3)H_(7)O_(2)N on reaction with iron and concentrated HCl gives a compound B of molecular formula C_(3)H_(9)N . Compound B on treatment with NaNO_(2) and HCl gives another compound C of molecular formula C_(3)H_(8)O . Compound C gives effervescence with sodium. On oxidation with CrO_(3) , compound C gives a saturated aldehyde containing three carbon atoms. Deduce the structures of compounds A, B and C and write the equations of the reactions involved.