When is the reduce method first called in a MapReduce job?

A.
Reduce methods and map methods all start at the beginning of a job, in order to provide
optimal performance for map-only or reduce-only jobs.

B.
Reducers start copying intermediate key value pairs from each Mapper as soon as it has
completed. The reduce method is called as soon as the intermediate key-value pairs start to
arrive.

C.
Reducers start copying intermediate key-value pairs from each Mapper as soon as it has
completed. The reduce method is called only after all intermediate data has been copied and
sorted.

D.
Reducers start copying intermediate key-value pairs from each Mapper as soon as it has
completed. The programmer can configure in the job what percentage of the intermediate data
should arrive before the reduce method begins.

Explanation:
In a MapReduce job reducers do not start executing the reduce method until the all
Map jobs have completed. Reducers start copying intermediate key-value pairs from the mappers
as soon as they are available. The programmer defined reduce method is called only after all the
mappers have finished.
Reference:24 Interview Questions & Answers for Hadoop MapReduce developers,When is the
reducers are started in a MapReduce job?
http://www.fromdev.com/2010/12/interview-questions-hadoop-mapreduce.html(question no. 17)