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: OBJECTIVE : Maternal tobacco smoking has negative effects on fetal growth .
### Output Text:
OBJECTIVE : Maternal tobacco <span class="chemical">smoking</span> has negative effects on fetal growth . 