What data does a Reducer reduce method process?

What data does a Reducer reduce method process?
A . All the data in a single input file.
B . All data produced by a single mapper.
C . All data for a given key, regardless of which mapper(s) produced it.
D . All data for a given value, regardless of which mapper(s) produced it.

Answer: C

Explanation:

Reducing lets you aggregate values together. A reducer function receives an iterator of input values from an input list. It then combines these values together, returning a single output value.

All values with the same key are presented to a single reduce task.

Reference: Yahoo! Hadoop Tutorial, Module 4: MapReduce