What are two possible reasons for this wait event?

You are managing a three-Instance RAC database. In the AWR report, you notice a gc current
block busy wait event on one of the database Instances, in the cluster top timed events section.
What are two possible reasons for this wait event?

A.
Access to blocks was delayed as the blocks were pinned In exclusive mode for updates by
some sessions.

B.
Access to blocks was delayed by log writes on remote Instance.

C.
Frequently used select statements are causing high disk Input/output contention.

D.
CPU shortages.

Explanation:
Analyzing Performance Using GCS and GES Statistics
The gc current block busy wait event indicates that the access to cached data blocks was delayed
because they were busy either in the remote or the local cache. This could be caused by any of
the following:
The blocks were pinned
The blocks were held up by sessions
The blocks were delayed by a log write on a remote instance
A session on the same instance was already accessing a block which was in transition between
instances and the current session needed to wait behind it (for example, gc current block busy)
Contention-Related Wait Events
The main wait events for contention-related waits are:
gc current block busy
gc cr block busy
gc buffer busy acquire/release
The contention-related wait event statistics indicate that a block was received which was pinned
by a session on another node, was deferred because a change had not yet been flushed to disk or
because of high concurrency, and therefore could not be shipped immediately. A buffer may also
be busy locally when a session has already initiated a cache fusion operation and is waiting for its

completion when another session on the same node is trying to read or modify the same data.
High service times for blocks exchanged in the global cache may exacerbate the contention, which
can be caused by frequent concurrent read and write accesses to the same data.
The gc current block busy and gc cr block busy wait events indicate that the local instance that is
making the request did not immediately receive a current or consistent read block. The term busy
in these events’ names indicates that the sending of the block was delayed on a remote instance.
For example, a block cannot be shipped immediately if Oracle Database has not yet written the
redo for the block’s changes to a log file.
Oracle Real Application Clusters Administration and Deployment Guide
11g Release 2 (11.2)