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 mean hemoglobin ( Hb ) levels were significantly declined in all patients from baseline of 14 . 2 g / dl to 14 . 0 g / dl , 13 . 5 g / dl , 13 . 2 g / dl and 12 . 7 g / dl at 1 , 2 , 3 and 6 months post - CAB , respectively .
### Output Text:
RESULTS : The mean hemoglobin ( Hb ) levels were significantly declined in all patients from baseline of 14 . 2 g / dl to 14 . 0 g / dl , 13 . 5 g / dl , 13 . 2 g / dl and 12 . 7 g / dl at 1 , 2 , 3 and 6 months post - CAB , respectively . 