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: We selected controls , matched on age and facility of usual care , at random from healthy members of the health plan .
### Output Text:
We selected controls , matched on age and facility of usual care , at random from healthy members of the health plan . 