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 : A randomised , double - blind study to compare the gastrointestinal ( GI ) tolerability , safety and efficacy of etoricoxib and diclofenac in patients with rheumatoid arthritis ( RA ) .
### Output Text:
OBJECTIVE : A randomised , double - blind study to compare the gastrointestinal ( GI ) tolerability , safety and efficacy of <span class="chemical">etoricoxib</span> and <span class="chemical">diclofenac</span> in patients with rheumatoid arthritis ( RA ) . 