The correct answer is: B. applications typically implement the mapper and reducer interfaces to provide the map and reduce methods.
A mapper maps input key/value pairs to a set of intermediate key/value pairs. This is true, but it is not the only thing that a mapper does. A mapper also has to provide the map method, which is responsible for transforming the input key/value pairs into the intermediate key/value pairs.
A reducer merges all intermediate values associated with the same intermediate key. This is also true, but it is not the only thing that a reducer does. A reducer also has to provide the reduce method, which is responsible for combining the intermediate values associated with the same intermediate key.
The mapper and reducer interfaces form the core of the job. This is not true. The mapper and reducer interfaces are just two of the many components that make up a Hadoop job. Other components include the job scheduler, the task scheduler, the data transfer process, and the file system.
Therefore, the correct answer is: B. applications typically implement the mapper and reducer interfaces to provide the map and reduce methods.