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: Median time to CR when RD was given as 2nd or > 2 ( nd ) - line treatment at 4 and 11 months , respectively .
### Output Text:
Median time to CR when <span class="chemical">RD</span> was given as 2nd or > 2 ( nd ) - line treatment at 4 and 11 months , respectively . 