Which of the following logic gates is a universal gate?

According to a graph drawn between the input and output voltages of a transistor connected in common emitter configuration, the region in which transistor acts as a switch is

If the energy gap of a semiconductor used for the fabrication of an LED is nearly 1.9 eV , then the color of the light emitted by the LED is

$10^{10}$ electrons enter the emitter of a junction transistor in a time of $0.4 \mu \mathrm{~s}$. If $5 \%$ of the electrons are lost in the base, then the collector current is