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 progression - free survivals ( PFSs ) were 2 . 6 ( stratum 1 ) and 2 . 7 months ( stratum 2 ) .
### Output Text:
Median progression - free survivals ( PFSs ) were 2 . 6 ( stratum 1 ) and 2 . 7 months ( stratum 2 ) . 