When a common emitter transistor is acting as an amplifier,

• A

  emitter - base junction is reverse biased and collector - emitter junction is forward biased.

• B

  current amplification is $\alpha \frac{R_c}{R_i}$

• C

  power amplification is $\beta {\left(\frac{R_c}{R_i}\right)}^2$

• D

  emitter - base junction is forward biased and collector emitter junction is reverse biased.