An AM signal is detected using an envelope detector. The carrier frequency and modulating signal frequency are $1 \; \mathrm{MHz}$ and $2 \; \mathrm{kHz}$ respectively. An appropriate value for the time constant of the envelope detector is
- $500 \; \mu \mathrm{sec}$
- $20 \; \mu \mathrm{sec}$
- $0.2 \; \mu \mathrm{sec}$
- $1 \; \mu \mathrm{sec}$