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: RESULTS : The incidence of pain was significantly lower in Groups R4 and R6 than in the control and R2 groups ( 12 / 32 and 6 / 31 vs 26 / 31 and 25 / 32 , respectively , P < 0 . 001 ) .
### Output Text:
RESULTS : The incidence of pain was significantly lower in Groups R4 and R6 than in the control and R2 groups ( 12 / 32 and 6 / 31 vs 26 / 31 and 25 / 32 , respectively , P < 0 . 001 ) . 