To use a transistor as an amplifier,which of the following conditions must be satisfied?

$A$ transistor is used as an amplifier in $CB$ mode with a load resistance of $5 \, k\Omega$. The current gain of the amplifier is $0.98$ and the input resistance is $70 \, \Omega$. The voltage gain and power gain respectively are:

For a transistor in the active region:
$(a)$ Base,emitter,and collector regions should have similar size and doping concentrations.
$(b)$ The base region must be very thin and lightly doped.
$(c)$ The emitter-base junction is forward-biased and the base-collector junction is reverse-biased.
$(d)$ Both the emitter-base junction and the base-collector junction are forward-biased.
Which one of the following pairs of statements is correct?

$A$ transistor has a voltage gain '$A$'. If a fraction '$\beta$' of its output is fed back to the input of the transistor,then the transistor becomes an oscillator when:

Three amplifiers,each having a voltage gain of $10$,are connected in series. The resultant gain would be

For a transistor to act as a switch,it must be operated in