How Is Bus Snooping Implemented to Ensure Cache Coherence

Multiple Choice

How is bus snooping implemented to ensure cache coherence?

A) The system maintains a centralized directory that records the items in each processor's cache. On a cache write, the writing processor "snoops" this list to determine whether the item is in other processors' caches, and if so, tells these other processors to remove the item.
B) The system only allows one processor at a time to cache each data item; when a processor tries to cache a data item, it "snoops" other processors' caches over the bus to determine whether it is allowed to cache the data.
C) Each processor "snoops" the processor-memory bus to determine whether another processor has sent a message about an update to a data item stored in its cache.
D) Each processor "snoops" the bus to determine whether a requested write from another processor is for a data item in the processor's cache. If the data resides in the processor's cache, the processor removes the data item from its cache.