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: Subjects were asked to aim at the centre of a target , subdivided in concentric circles , from 5 m distance .
### Output Text:
Subjects were asked to aim at the centre of a target , subdivided in concentric circles , from 5 m distance . 