What is the difference between a failed task attempt and a killed task
attempt?

*A.  *A failed task attempt is a task attempt that threw an unhandled
exception. A killed task attempt is one that was terminated by the
JobTracker.

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*B.  *A failed task attempt is a task attempt that did not generate any key
value pairs. A killed task attempt is a task attempt that threw an
exception, and thus killed by the execution framework.

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*C.  *A failed task attempt is a task attempt that completed, but with an
unexpected status value. A killed task attempt is a duplicate copy of a
task attempt that was started as part of speculative execution.

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*D.  *A failed task attempt is a task attempt that threw a RuntimeException
(i.e., the task fails). A killed task attempt is a task attempt that threw
any other type of exception (e.g., IOException); the execution framework
catches these exceptions and reports them as killed.