Which two statements are true about analog to digital conversion of voice signals for use in digital telephony networks？ （）

A. The three required steps in the analog to digital conversion are sampling， encoding， and compression.
B. The three required steps in the analog to digital conversion are sampling， quantization， and compression.
C. The three required steps in the analog to digital conversion are sampling， quantization， and encoding.
D. The output of the sampling process is a pulse code modulation （PCM） signal.
E. The output of the sampling process is a pulse amplitude modulation （PAM） signal.