21. The input impedance of a common emitter NPN transistor is the between
A. the variation in emitter-base potential and the corresponding variation in the base current at constant collector-emitter potential
B. the variation in collector current to the corresponding variation in emitter current at constant collector base potential
C. the variation in the collector current to the corresponding change in the collector base voltage at constant emitter current
D. the variation in the emitter current to the corresponding variation in collector current at a constant collector emitter voltage