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: Male rats were treated with MA ( 10 mg / kg , every 2 h for four injections ) .
### Output Text:
Male rats were treated with <span class="chemical">MA</span> ( 10 mg / kg , every 2 h for four injections ) . 