In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: SEARCH STRATEGY : We searched the following databases up to November 2004 : the Cochrane Menstrual Disorders and Subfertility Group Trials Register , Cochrane Central Register of Controlled Trials ( CENTRAL ) , MEDLINE , EMBASE , Biological Abstracts .
### Output Text:
SEARCH STRATEGY : We searched the following databases up to November 2004 : the Cochrane Menstrual Disorders and Subfertility Group Trials Register , Cochrane Central Register of Controlled Trials ( CENTRAL ) , MEDLINE , EMBASE , Biological Abstracts . 