How are DTMF digits transported in RFC 2833?

How are DTMF digits transported in RFC 2833?
A . In the RTP stream with the named telephone events payload format.
B . In the RTP stream with the regular audio payload format.
C . In SIP NOTIFY messages.
D . In SIP INFO messages.
E . In SIP SUBSCRIBE messages.

Answer: A

Explanation:

DTMF digits and named telephone events are carried as part of the audio stream, and MUST use the same sequence number and time-stamp base as the regular audio channel to simplify the generation of audio waveforms at a gateway. The default clock frequency is 8,000 Hz, but the clock frequency can be redefined when assigning the dynamic payload type.