Which one of the following classes must you extend in o…

Which one of the following classes must you extend in order to implement a custom indexer?

A.
Mage_Eav_Model_Indexer_Abstract

B.
Mage_Core_Model_lndexer_Abstract

C.
Mage_Index_Model_Entity_Indexer_Abstract

D.
Mage_Index_Model_Indexer_Abstract