Which two statements are true about analog to digital conversion of voice signals for use in digital telephony networks？（）
- AThe three required steps in the analog to digital conversion are sampling，encoding，and compression.
- BThe three required steps in the analog to digital conversion are sampling，quantization，and compression.
- CThe three required steps in the analog to digital conversion are sampling，quantization，and encoding.
- DThe output of the sampling process is a pulse code modulation （PCM） signal.
- EThe output of the sampling process is a pulse amplitude modulation （PAM） signal.