One mole of a mixture of `N_(2),NO_(2)` and `N_(2)O_(4)`, has a mean molar mass of `55.4`. On heating to a temperature at which `N_(2)O_(4)` may be dissociated `:N_(2)O_(4)rarr2NO_(2)`, the mean molar mass tends to the lower value of `39.6`. What is the mole ratio of `N_(2) : NO_(2) : N_(2)O_(4)` in the original mixture?
A. `0.5 :0.1:0.4`
B. `0.6:0.1:0.3`
C. `0.5 : 0.2 :0.3`
D. `0.6 : 0.2 : 0.2`
