In the reducer, the MapReduce API provides you with an iterator over Writable values. What does calling the next () method return? 

It returns a reference to a different Writable object time. 

It returns a reference to a Writable object from an object pool. 

It returns a reference to the same Writable object each time, but populated with different data. 

It returns a reference to a Writable object. The API leaves unspecified whether this is a reused object or a new object. 

It returns a reference to the same Writable object if the next value is the same as the previous value, or a new Writable object otherwise.