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: Women who were born between 1971 and 1975 had the highest rate of rubella nonimmunity ( 9 . 5 % ) .
### Output Text:
Women who were born between 1971 and 1975 had the highest rate of rubella nonimmunity ( 9 . 5 % ) . 