What mechanism is used to maintain time synchronization in an 802.11 BSS?

A.
Stations use ACK frames to regularly reset their internal clock to match that of their AP.

B.
When receiving Beacon frames, stations check and correct their clocks using the included timestampfiel

C.
C. Access points synchronize time information from an external time-source using NTP.

D.
A continuous heartbeat frame (used specifically for time synchronization) is sent from access points to associated stations.