You need to perform statistical analysis in your MapReduce job and would like to call methods in
the Apache Commons Math library, which is distributed as a 1.3 megabyte Java archive (JAR) file.
Which is the best way to make this library available to your MapReducer job at runtime?

A.
Have your system administrator copy the JAR to all nodes in the cluster and set its location in
the HADOOP_CLASSPATH environment variable before you submit your job.

B.
Have your system administrator place the JAR file on a Web server accessible to all cluster
nodes and then set the HTTP_JAR_URL environment variable to its location.

C.
When submitting the job on the command line, specify the –libjars option followed by the JAR
file path.

D.
Package your code and the Apache Commands Math library into a zip file named JobJar.zip

Explanation:
The usage of the jar command is like this,
Usage: hadoop jar <jar> [mainClass] args…
If you want the commons-math3.jar to be available for all the tasks you can do any one of
these
1. Copy the jar file in $HADOOP_HOME/lib dir
or
2. Use the generic option -libjars.