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: ORR was 4 . 7 % ( 7 . 1 % stratum 1 ; 2 . 3 % stratum 2 ) .
### Output Text:
ORR was 4 . 7 % ( 7 . 1 % stratum 1 ; 2 . 3 % stratum 2 ) . 