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: Daily subject symptom cards were completed during a screening period to assess pretreatment symptoms and during a 4 - week treatment period while subjects received study medications .
### Output Text:
Daily subject symptom cards were completed during a screening period to assess pretreatment symptoms and during a 4 - week treatment period while subjects received study medications . 