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 duration of response was 34 . 4 months , and it was higher in patients who received RD until progression ( not reached versus 19 months , p < 0 . 001 ) .
### Output Text:
Median duration of response was 34 . 4 months , and it was higher in patients who received <span class="chemical">RD</span> until progression ( not reached versus 19 months , p < 0 . 001 ) . 