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: On average , MA abusers had 7 . 8 % smaller hippocampal volumes than control subjects ( p < 0 . 01 ; left , p = 0 . 01 ; right , p < 0 . 05 ) and significant white - matter hypertrophy ( 7 . 0 % ; p < 0 . 01 ) .
### Output Text:
On average , <span class="chemical">MA</span> abusers had 7 . 8 % smaller hippocampal volumes than control subjects ( p < 0 . 01 ; left , p = 0 . 01 ; right , p < 0 . 05 ) and significant white - matter hypertrophy ( 7 . 0 % ; p < 0 . 01 ) . 