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 : Diabetic nephropathy is one of the major causes of renal failure , which is accompanied by the production of reactive oxygen species ( ROS ) .
### Output Text:
OBJECTIVE : Diabetic nephropathy is one of the major causes of renal failure , which is accompanied by the production of reactive <span class="chemical">oxygen</span> species ( ROS ) . 