Cost (%CPU) computation in the explain

Hi

   How the cost and %CPU calculated in the explain plan.  In the below example the query is returning 1 row but the %CPU is showing 100%. 

   Request to explain the basis of %CPU computation

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| Id  | Operation                   | Name             | Rows  | Bytes | Cost (%CPU)| Time     |
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|   0 | SELECT STATEMENT            |                  |       |       |     2 (100)|          |
|*  1 |  TABLE ACCESS BY INDEX ROWID| XXXX             |     1 |   303 |     2   (0) | 00:00:01 |
|*  2 |   INDEX UNIQUE SCAN         | XXXX_idx         |     1 |       |     1   (0) |  00:00:01|
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Regards

Bhanu Pratap M