QUESTION $4$

The external quantum efficiency, ${}_{\text{e}\text{x}\text{t}},$ of an LED is always less than the internal quantum efficiency, ${}_{\text{i}\text{n}\text{t}},$ because:

A$.$

Only light reaching the interface at an angle of incidence ${}_{in}$ less than the critical angle ${}_c$ has the potential to be transmitted

B$.$

Light incident with ${}_{in}{}_C$ is totally reflected and do not exit.

Even some of the light which is not totally reflected