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: Antitumor efficacy of MC versus AC was comparable : objective response rates , 43 % versus 43 % ; median time to progression , 5 . 1 % versus 5 . 5 months ; median time to treatment failure , 4 . 6 versus 4 . 4 months ; and median survival , 19 versus 16 months .
### Output Text:
Antitumor efficacy of MC versus AC was comparable : objective response rates , 43 % versus 43 % ; median time to progression , 5 . 1 % versus 5 . 5 months ; median time to treatment failure , 4 . 6 versus 4 . 4 months ; and median survival , 19 versus 16 months . 