When can a reduce class also serve as a combiner without affecting the output of a MapReduce
program?

A.
When the types of the reduce operation’s input key and input value match the types of the
reducer’s output key and output value and when the reduce operation is both communicative and
associative.

B.
When the signature of the reduce method matches the signature of the combine method.

C.
Always. Code can be reused in Java since it is a polymorphic object-oriented programming
language.

D.
Always. The point of a combiner is to serve as a mini-reducer directly after the map phase to
increase performance.

E.
Never. Combiners and reducers must be implemented separately because they serve different
purposes.

Explanation:
You can use your reducer code as a combiner if the operation performed is
commutative and associative.