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: NIN correlated with blood oxygen level - dependent ( BOLD ) activity levels in a midpontine site in the posterior basis pontis .
### Output Text:
NIN correlated with blood <span class="chemical">oxygen</span> level - dependent ( BOLD ) activity levels in a midpontine site in the posterior basis pontis . 