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: A significant relationship was observed between the rate of reduction of hemoglobin concentrations at week 2 and the severity of anemia ( P < 0 . 01 ) .
### Output Text:
A significant relationship was observed between the rate of reduction of hemoglobin concentrations at week 2 and the severity of anemia ( P < 0 . 01 ) . 