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: Postnatal mortality was increased among experimental animals , and arterial oxygen saturation was 96 + / - 1 % in 1 - day - old control animals and significantly lower ( P < 0 . 01 ) in fluoxetine - exposed pups ( 79 + / - 2 % ) .
### Output Text:
Postnatal mortality was increased among experimental animals , and arterial <span class="chemical">oxygen</span> saturation was 96 + / - 1 % in 1 - day - old control animals and significantly lower ( P < 0 . 01 ) in <span class="chemical">fluoxetine</span> - exposed pups ( 79 + / - 2 % ) . 