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: World Health Organization Grade 3 - 4 neutropenia and thrombocytopenia occurred in 39 . 9 % and 11 . 4 % of patients , respectively .
### Output Text:
World Health Organization Grade 3 - 4 neutropenia and thrombocytopenia occurred in 39 . 9 % and 11 . 4 % of patients , respectively . 