how to replace mage_page_block_html_head

Choose the correct answer from the following

Which one of the following xpaths is correct for replacing Mage_Page_Block_Html_Head with Custom_Module_Block_Head via the block factory method?
a. global/rewrite/blocks/page/html/head
b. global/blocks/page/html_head
c. global/blocks/rewrite/page/head
d. global/blocks/page/rewrite/html_head
e. global/page/blocks/rewrite/html_head

the right answer is :

global/blocks/page/rewrite/html_head