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: Before and 0 . 5 , 1 , 2 , 3 , and 6 h after injection the subjects were given attention and mnemonic tests .
### Output Text:
Before and 0 . 5 , 1 , 2 , 3 , and 6 h after injection the subjects were given attention and mnemonic tests . 