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: DESIGN , SETTING , AND PARTICIPANTS : Population - based , multisite , case - control study of women who had pregnancies affected by 1 of more than 30 eligible major birth defects identified via birth defect surveillance programs in 10 states ( n = 13 155 ) and control women randomly selected from the same geographical regions ( n = 4941 ) .
### Output Text:
DESIGN , SETTING , AND PARTICIPANTS : Population - based , multisite , case - control study of women who had pregnancies affected by 1 of more than 30 eligible major birth defects identified via birth defect surveillance programs in 10 states ( n = 13 155 ) and control women randomly selected from the same geographical regions ( n = 4941 ) . 